Use (civ-specific) standards to show formations.

Since we can have battalion-style formations (r25939 / rP25939), it is nice to show a TW-style banner for them.
Code adapted from @Silier at https://wildfiregames.com/forum/topic/68008-formation-standards-total-war-style-flags-over-battalions/.

Differential revision: https://code.wildfiregames.com/D4467
Comments by: @asterix, @Silier
Refs. #4545