Method to community label a reblog

[marcustyphoon]

Concept

As discussed in a thread with our good friend echo, the tumblr API allows one to community label a reblog even if it doesn't have any content (using it not as an indication that the reblog's added content is mature, but instead as a way for followers and downstream rebloggers to have the content labelled without upstream versions also being labelled), but the post form does not. The community label system generally has a fair bit of nuance, which I won't get into more for brevity and relevance reasons.

Anyway, it could be neat if XKit Rewritten allowed one to do this. This could either be part of the Quick Reblog form if one enables a preference (although there's not a ton of room?), which I could build if we think that's the right path, or it could just be done as an action after the user performs the reblog, which I already built (#827).