Simplified image sensitivity label

this changes it from

[_] Attachments NOT marked as sensitive
[v] Attachments marked as sensitive

to less confusing and less jumpy

[_] Mark attachments as sensitive
[v] Mark attachments as sensitive

changed the description

I suppose just:

• NSFW

or

• Sensitive

I think it should still contain the action, i.e. "mark attachments...", leaving it just "Sensitive" could be confusing.

Another option would be "Attachments are NSFW" or something like that. Without disappearing "not"

merged

mentioned in commit eb93034a