Rethink crossposting

[MightyAlex200]

Currently, crossposing just creates a link between a post and a tag the same way as when a post is created. This can cause a couple of issues:

1. In the solution to i18n proposed in Change subs from strings to integers #1, it would be possible to crosspost something like the "English" post to another tag, completely breaking it.
2. Anybody can just crosspost a post to an "nsfw" tag, hiding it for most users.
3. Crossposting something to the wrong tag will likely get it some downvotes from people who just don't want to see it on that specific tag.

In my opinion, the best solution to this problem would be to

1. Separate crosspost links and tag links
2. Vote on tags, not posts

Respectively, this would allow people to:

1. See who a post's intended target audience is
2. Vote on that posts fitness for that target audience