Add SQL DDL for rename requests

Add SQL table and index definitions for a new `renameuser_status` table
that will be used to store new requests, back a request processing tool
for stewards and block new account registrations from taking a name that
pending approval as a rename.

Expected queries:
; New account registration check (highest volume)
:* select count(*) where rq_newname = ... and rq_status = 'pending';
; On visit to request creation special page
:* select count(*) where rq_name = ... and rq_wiki = ... and rq_status = 'pending';
; On processing tool
:* select * where rq_id = ...;
:* select ... where rq_status = 'pending' order by ... limit N,25;
:* select ... where rq_status <> 'pending' order by ... limit N,25;
: The `order by` clauses will be a single field and not a blob.
: Default sort will be by `rq_request_timestamp` asc.

It's tempting to make a unique constraint on (rq_name, rq_wiki,
rq_status), but it is possible that a single (rq_name, rq_wiki) pair
could have multiple records with rq_status='rejected' if the user is
just really bad at picking names that the stewards approve of.

Bug: 68889
Change-Id: I61acceb4ea3bce5b03a1408c89f21559b928c623
Merge "Add SQL DDL for rename requests"