Migrate/obsolete incongruent Leadworks, EBR, and Rivtech guns

[tenmillimaster]

Summary

SUMMARY: Content "migrates+obsoletes firearms incongruent with lore"

Purpose of change

Leadworks largely takes a leaf from the XM8 program, which sought to integrate modularity into a service rifle, but not at a user level. The rifles, and other similar built-in-mod guns have not seen success in real life as they are purported to have done so in-game.
The RM360 describes an entirely fictional .22 LR gatling gun, which would be available to SOT's only. Seeing as it is mostly a range toy, I doubt a serious company would build such a thing and market it.
The Mosin EBR kits have been obsoleted by the mounting furniture gunmods.

Describe the solution

Obsoletes or migrates items in question. Redefines LW-Feed, obsoleted, to work with Stanags instead of migrated items. Backup revolver is obsoleted, to either be later migrated to an AR pistol or a single-shot 223 PAR-1.
460 guns migrated to a .460 1911, magazines altered to accept them.

Describe alternatives you've considered

Obsoleting .460 guns and adding 1911 later.
That whole leadworks western gun company I was suggesting, but that's still a lot of mental gymnastics.
Pretending that the XM8 was actually successful.

Testing

Loaded mainline. Loaded generic guns. Checked to make sure everything still migrated correctly. Figuring out how to handle migration/blacklisting in GG was a bit of a bear, but I managed.

Additional context

Generic guns has a potential load error- I have accidently re-used the abstract item ID pistol_revolver for the generic guns police revolver. Fix to come.

[I-am-Erk]

I do really want to bring Rivtech back with some other items. We should look into having them on item description snippets and things

[tenmillimaster]

I do really want to bring Rivtech back with some other items. We should look into having them on item description snippets and things

I haven't removed any Rivtech guns in their caseless families.

[I-am-Erk]

Ah good

[anothersimulacrum]

The items that no longer exist are still on blacklists in mods, and need to be removed, because it's causing the tests to fail.

[kevingranade]

I have those fixes locally, I think I'll push this and then push my fixes afterwards.