Post deletion stubs

[discomrade]

Is your feature request related to a problem? Please describe.
When a post is deleted, it is simply removed from the database and disk. This kind of deletion can give a bad user experience:

• If a duplicate post is deleted because a better quality version exists, the uploader may not understand why their post was removed, and anyone who has favorited/pooled/etc. the post, or who visits it from an external link, may be left confused. It would be useful to know where the higher-quality version is.
• If a post did break a rule, the user may not understand which rule was broken.

Describe the solution you'd like
Deleted posts are unlisted in searches and the file removed from disk, but the post kept in the database. A deletion message is shown on the view page, showing the deletion reason.

[shish]

Yes <3 (I'm sure this was requested already, but I can't find the previous issue)

[discomrade]

Ah you're right, I found this: #900