Allow unused 'Alien Sighted' narratives to be played properly again

[BlackDog86]

Fixes #1492

[BlackDog86]

This will require some reasonably extensive testing to ensure there are no unintended consequences before merge

[BlackDog86]

Confident this is good to go now :)

[Tedster59]

@BlackDog86 - looking back through this, does this also handle cases of modded enemies that reuse a narrative moment, but are a different character group? currently, the various custom mutons from mods such as Muton Harriers, or LW2/LWOTC, all play the base muton narrative on first sighting. Should they?

[BlackDog86]

To summarise -

Base game:

• Narrative moments are only played on the first sighting of a character group.
• Some base game narrative moments remain unplayed because m2/m3 variants are in the same group as the m1 variant.
• Adding a narrative moment to a modded enemy which is in an existing character group, will only play if it's the first unit sighted in that group.
• Adding a narrative to a modded enemy in a new character group, will cause the narrative moment to be played again, even if it's been used already.

Adjusted behaviour

• Narrative moments are played on the first sighting of a character template.
• This allows the unused narratives to be restored to m2/m3 variants of units.
• Adding a narrative moment to a modded enemy in an existing character group will play the narrative if it's not been played already.
• Adding an already played narrative to a modded enemy in a new character group will no longer play the narrative.

Some modded enemies which have already reused cut-content narrative moments will find that those no longer play, unless that unit was sighted before the base-game one (e.g. If I create a unit which reuses the lancer M2 moment, but it shows up at a higher force level than the M2 lancer, likely it will never play because the M2 lancer has already used it).

Answers on a postcard as to whether this is a problem or not.