Security fixes:
T197279, CVE-2019-12468: Directly POSTing to Special:ChangeEmail would allow
for bypassing reauthentication, allowing for potential account takeover.
T204729, CVE-2019-12473: Passing invalid titles to the API could cause a DoS
by querying the entire `watchlist` table.
T207603, CVE-2019-12471: Loading user JavaScript from a non-existent account
allows anyone to create the account, and XSS the users' loading that script.
T208881: blacklist CSS var().
T199540, CVE-2019-12472: It is possible to bypass the limits on IP range
blocks (`$wgBlockCIDRLimit`) by using the API.
T212118, CVE-2019-12474: Privileged API responses that include whether a
recent change has been patrolled may be cached publicly.
T209794, CVE-2019-12467: A spammer can use Special:ChangeEmail to send out
spam with no rate limiting or ability to block them.
T25227, CVE-2019-12466: An account can be logged out without using a token(CRRF)
T222036, CVE-2019-12469: Exposed suppressed username or log in Special:EditTags.
T222038, CVE-2019-12470: Exposed suppressed log in RevisionDelete page.
T221739, CVE-2019-11358: Fix potential XSS in jQuery.