Add a user_actions view. This view unions together a bunch of tables to
produce an event log of every action taken by a user.
Also add a bunch of indexes to make queries on this table efficient.
Even though the view is an enormous query combining together about 30
different tables, queries are very efficient as long as every table has
`created_at` and `(user_id, created)` indexes.
Add a new IP address search page at /ip_addresses. Replaces the old
search page at /moderator/ip_addrs.
On user profile pages, show the user's last known IP to mods. Also add
search links for finding other IPs or accounts associated with the user.
IP address search uses a big UNION ALL statement to merge IP addresses
across various tables into a single view. This makes searching easier,
but is known to timeout in certain cases.
Fixes#4207 (the new IP search page supports searching by subnet).
