RELEASE-NOTES: Use New/Changed/Removed pattern for Configuration section

RELEASE-NOTES: Use New/Changed/Removed pattern for Configuration section

This was already used for external libraries. This commit
changes the order to be consistently 'New/Changed/Removed',
and adopts the pattern for configuration changes as well.

For improved scannability, the bullet points now start with
the name of the configuration setting(s), followed by a sentence,
with an optional ticket in brackets after the sentence(s).

* A number of bullet points under "Configuration changes" were
  in fact, not configuration changes. These have been moved to
  "New features" or "Other changes" for now.

* Add mention of the relevant configuration variable to some
  of the release notes: $wgTidyDriver, `watchlistdays`,
  $wgGroupPermissions, $wgGroupPermissions.

Also fix ReleaseNotesTest to count characters, not bytes,
this was causing it to count – as two, and € as three.

Change-Id: Ie89dac6408f8a8dafbf59efe73a11f4d282c0c6b