Atlas default player settings suboptimal for RMS

[FeXoR]

Error description: Random maps don't generate properly (in some circumstances)

Reason: Not all players civilizations are set

To reproduce:

• Open Atlas
• Reduce the number of players to 2
• Generate "Fortress" random map (working, all players beyond 2 will now have no civ set)
• Increase the number of player to 3 (player 3 will still have no civ)
• Generate "Fortress" random map (crashing)

This is due to player 3 will have no civ defined. On other maps that "only" leads to warnings about that and player 3 will get Athenian starting entities despite it's civ (which is undefined ofc.).

How to fix: I recommend to check all buttons for "civilization" in Atlas player settings by default (even if the number of player are reduced, a random map is generated and then the number of players is increased again). That is the initial state (if the number of players are NOT changed) anyway. Additionally I'd suggest to force all players civilizations to be set if the "Generate map" button is pressed (because RMS needs set civs - as is and in some cases anyway, see below).

(OPTIONAL: If it's wanted to generate maps with starting entities without a preset civ to enable the use of RMS as a basis of Skirmish Maps starting entities for the case civ = "random" (I guess) needs to be added (I don't know where). However, due to walls not having the same length for all civs and civs have different NUMBER of starting entities (e.g. Iberian civ bonus walls) RMS with walls will never be possible that way.)

[FeXoR]

May be related to #1872

A fallback to Athens and a warning (handle the problem more graciously) could still be good for RMS.

For adding a "scirm" civ (like in 0ad/binaries/data/mods/public/simulation/templates/skirmish/units/special_starting_unit.xml) a skirm.json would need to be added in 0ad/binaries/data/mods/public/civs (if RMS are wanted as basis of generating Skirmish maps in Atlas).