feat(arsenal): Improved loadout migration

[WitchFreya]

Describe your changes

• Moved loadout migration feature responsibility back to this mod.
• Migrations now test whether or not one exists when selecting to enable/disable the button.
• Migrations can work with non-null items that just aren't in the arsenal (such as when the scope of something has changed).
• Migrations now have a confirmation feature listing exactly what they'll change.
• When migrated, the new version will have the original name and a backup will be saved.
• Loadouts with migrations available will be yellow instead of red/grey.
• Loadouts with unavailable or null items will indicate how many on hover.

Screenshots (if appropriate)