Drop the creator_id and updater_id fields from wiki pages. These fields
had several issues:
* The creator_id field was inconsistent with the wiki_page_versions
table. Apparently during the migration to Danbooru 2 in 2012-2013 the
creator_id field got reset to whoever last updated the wiki at that
point in time.
* Saving a wiki would set the updater_id even when nothing actually
changed. This also caused the updated_at timestamp to get bumped.
Because of this, anything that saved a wiki, including things like
creating aliases or implications, would bump the updater_id and
updated_at even though the wiki didn't actually change. This meant
these fields weren't consistent with the wiki_page_versions history.
Changes:
* Remove `creator_name` field from the /wiki_pages.json API.
* Remove creator name search option from /wiki_pages/search.
Add a dtext_links table for tracking links between wiki pages. This is
to allow for broken link detection and "what links here" searches, among
other uses.
Remove all infrastructure around approving or rejecting user name
changes. Name changes haven't been moderated for several years.
* Remove status, approver_id, change_reason, and rejection_reason fields.
* Remove approve and reject controller actions.
Fixup for 9c34d5cc3. Default to member level in the database because
setting it in `customize_new_user` clobbered user levels set by factory
bot in tests, which broke the test suite.
* Change the source index on posts from `(lower(source) gin_trgm_ops) WHERE source != ''`
to just `(source gin_trgm_ops)`. The WHERE clause prevented the index
from being used in source:<url> searches because we didn't specify
the `source != ''` clause in the search itself. Excluding blank
sources only saved a marginal amount of space anyway. This fixes
timeouts in source:<url> searches and in the bookmarklet (since we do
a source dupe check on the upload page too).
* Also switch from indexing `lower(name)` to `name` on pools and users.
We don't need to lowercase the column because GIN indexes can be used
with both LIKE and ILIKE queries.
* Drop support for `source:pixiv/artist-name` searches. This was a hack
that only worked on old pixiv urls that haven't been used for years.
* Replace the old SourcePattern(lower(source)) index with a trigram index.
