Add version field to campaign descriptor file.

This is used to provide a UI hint to guide players towards campaigns
that have been updated to work with the current version of the game.
All the campaigns we currently have were made for an unknown version of
the game, so they're all flagged as incompatible.

The version field is not the DCS Liberation version number because the
campaign format may change multiple times during development. Instead
the version number is a monotonically increasing integer that we
increment whenever a game change requires campaign updates.

resources/ui/templates/campaigntemplate_EN.j2

@@ -1,8 +1,24 @@
-<strong>Author(s): {{ campaign.authors }}</strong>
-<br/><br/>
+<p><strong>Author(s): {{ campaign.authors }}</strong></p>
 
-<strong>Default factions:</strong>
-<span style="color:#82A466">&nbsp;{{campaign.recommended_player_faction}}</span> VS <span style="color:orange">&nbsp;{{campaign.recommended_enemy_faction}}</span>
-<br/>
+{% if not campaign.version %}
+<p><strong style="color:orange">This campaign was created for an unknown version
+of the game.</strong></p>
+<p>You can still attempt to play this campaign but there may be game breaking
+{% elif campaign.is_out_of_date %}
+<p><strong style="color:orange">This campaign was created for an older version
+of the game.</strong></p>
+<p>You can still attempt to play this campaign but there may be game breaking
+bugs.</p>
+{% elif campaign.is_from_future %}
+<p><strong style="color:orange">This campaign was created for a newer version
+of the game.</strong></p>
+<p>You can still attempt to play this campaign but there may be game breaking
+bugs.</p>
+{% else %}
+<p><strong style="color:#82A466">This campaign is up to date.</strong></p>
+{% endif %}
 
-{{ campaign.description|safe }}
+<p><strong>Default factions:</strong></p>
+<p><span style="color:#82A466">{{campaign.recommended_player_faction}}</span> VS <span style="color:orange">&nbsp;{{campaign.recommended_enemy_faction}}</span></p>
+
+{{ campaign.description|safe }}