Add "force content warning" setting for user moderation (resolves #905)

What does this MR do?

Adds a new "Force content warning" field that can be applied by moderators to local and remote users. The value entered here will be automatically attached as a content warning to all new and edited posts by the target user. This allows instance staff to selectively filter problem users by applying a descriptive CW.

As this is a potentially dangerous tool, all changes to a forced content warning are recorded in the moderation log.

Additionally:

• Fix lint errors in NoteCreateService and NoteEditService.
• Fix lint errors in modlog.ModLog.vue.
• Add missing translation entry for ModLog "Declined" events.
• Immediately clear memory caches when setting "Mark all media as NSFW" for a user.

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