Add plannable tankers.

This Pull Request lets users plan Tanker flights.

Features:

- Introduction of `Refueling` flight type.
- Tankers can be purchased at airbases and carriers.
- Tankers get planned by AI.
- Tankers are planned from airbases and at aircraft carriers.
- Tankers aim to be at high, fast, and 70 miles from the nearest threat.
  (A10s won't be able to tank)
- Tankers racetrack orbit for one hour.
- Optional Tickbox to enable legacy tankers.
- S-3B Tanker added to factions.
- KC-130 MPRS added to factions.
- Kneeboard shows planned tankers, their tacans, and radios.

Limitations:

- AI doesn't know whether to plan probe and drogue or boom refueling
  tankers.
- User can't choose tanker speed.  Heavily loaded aircraft may have
  trouble.
- User can't choose tanker altitude.  A-10s will not make it to high
  altitude.

Problems:

- Tanker callsigns do not increment, see attached image.  (Investigated:
  Need to use `FlyingType.callsign_dict`, instead of just
  `FlyingType.callsign`.  This seems like it might be significant work
  to do.).
- Having a flight of two or more tankers only spawns one tanker.
- Let me know if you have a solution, or feel free to commit one.

https://user-images.githubusercontent.com/74509817/120909602-d7bc3680-c633-11eb-80d7-eccd4e095770.png

resources/units/unit_info_text.json

@@ -422,6 +422,36 @@
             "year-of-variant-introduction": "1995"
         }
     }],
+    "KC130": [{
+        "default": {
+            "name": "KC-130",
+            "text": "The Lockheed Martin (previously Lockheed) KC-130 is a family of the extended-range tanker version of the C-130 Hercules transport aircraft modified for aerial refueling.",
+            "country-of-origin": "USA",
+            "manufacturer": "Lockheed Martin",
+            "role": "Tanker",
+            "year-of-variant-introduction": "1962"
+        }
+    }],
+    "KC-135": [{
+        "default": {
+            "name": "KC-135 Stratotanker",
+            "text": "The Boeing KC-135 Stratotanker is a military aerial refueling aircraft that was developed from the Boeing 367-80 prototype, alongside the Boeing 707 airliner.",
+            "country-of-origin": "USA",
+            "manufacturer": "Beoing",
+            "role": "Tanker",
+            "year-of-variant-introduction": "1957"
+        }
+    }],
+    "KC135MPRS": [{
+        "default": {
+            "name": "KC-135 Stratotanker MPRS",
+            "text": "The Boeing KC-135 Stratotanker is a military aerial refueling aircraft that was developed from the Boeing 367-80 prototype, alongside the Boeing 707 airliner.  This model has the Multi-point Refueling System modification, allowing for probe and drogue refuelling.",
+            "country-of-origin": "USA",
+            "manufacturer": "Beoing",
+            "role": "Tanker",
+            "year-of-variant-introduction": "1994"
+        }
+    }],
     "L-39C": [{
         "default": {
             "name": "L-39C Albatros",
@@ -743,6 +773,16 @@
             "year-of-variant-introduction": "1984"
         }
     }],
+    "S-3B Tanker": [{
+            "default": {
+            "name": "S-3B Tanker",
+            "text": "The Lockheed S-3 Viking is a 4-crew, twin-engine turbofan-powered jet aircraft that was used by the U.S. Navy (USN) primarily for anti-submarine warfare. In the late 1990s, the S-3B's mission focus shifted to surface warfare and aerial refueling. The Viking also provided electronic warfare and surface surveillance capabilities to a carrier battle group. A carrier-based, subsonic, all-weather, long-range, multi-mission aircraft, it carried automated weapon systems and was capable of extended missions with in-flight refueling. Because of its characteristic sound, it was nicknamed the \"War Hoover\" after the vacuum cleaner brand. The S-3 was phased out from front-line fleet service aboard aircraft carriers in January 2009, with its missions taken over by aircraft like the P-3C Orion, P-8 Poseidon, Sikorsky SH-60 Seahawk and Boeing F/A-18E/F Super Hornet",
+            "country-of-origin": "USA",
+            "manufacturer": "Lockheed",
+            "role": "Carrier-based Tanker",
+            "year-of-variant-introduction": "1984"
+        }
+    }],
     "SA342L": [{
         "default": {
             "name": "SA 342L Gazelle",