Avoid doing one SQL query per topic when checking for new topics on the
forum index.
This also changes it so that forum topics aren't always marked as new
for anonymous users.
Move the parsing for the [bur:<id>], [ta:<id>], [ti:<id>] pseudo tags to
the main parser in `DText.format_text`. This fixes a bug where wiki
links inside bulk update requests on the forum weren't properly
colorized because the text of the BUR was embedded after we scanned for
wiki links, not before.
This also ensures that tags inside bulk update requests will be recorded
in the dtext_links table, meaning that forum posts can be properly
searched by tags.
This incidentally means that these request pseudo tags can now be used
outside the forum.
Bug: When editing a forum post (other than the OP) that contained a
dynamic request, the edit form didn't contain the [bur:1234] syntax.
Instead it contained the full body of the request.
This happened because `forum_post.body` was inadvertantly mutated while
rendering the request.
Fixes https://github.com/r888888888/danbooru/issues/3824#issuecomment-454884235.
