Monobook: Remove unused file FF2Fixes.css

As of Id8214c6a5 and Ia54dd738b (per bug 35906) we started
blacklisting Firefox 2 from the additional resource loader.

Though we haven't supported Firefox 2 for several years, Monobook
has had this patch for years and we never removed it (cheap to
maintain).

Since wikibits.js is no longer being loaded in Firefox 2, this
fix is now obsolete. It is never loaded, not even in Firefox 2.

It is causing problems because the legacy code incorrectly
detects Firefox 20 as Firefox 2, so instead of being a useless
file that is never loaded, it is actually causing problems by
being loaded in Firefox 20.

Bug: 47202
Change-Id: I2fdd0da8c17553a3465ef27c46a4632e135c4405
mediawiki.user: Use session ID instead of 1-year cross-session cookies

No cookies should be assigned for longer than session expiry.

Bug: 44327
Change-Id: If2f096dadb639769d859e1596d84b3ad5775a01d
SECURITY: Do checks on all upload types

Also, verify file before stashing it

Change-Id: I5dd027acc994cf7308c9e4d85a61237d802ccee8
Updated release notes and version number

Change-Id: I630010e6371ac4ba9ea24e5f2f48e891d3e41c2f
Merge branch 'REL1_20' of https://gerrit.wikimedia.org/r/p/mediawiki/core into REL1_20