Allow modification of flags on Recent Changes

Moved the message map in ChangesList::flag to a new setting named
$wgRecentChangesFlags and used it keys to iterate over the flags
Also allow setting the class to deal with minor vs. minoredit

Also make ChangesList::recentChangesFlags public to be used from
extensions, when they will build its own line

Changed one list of placeholders to use recentChangesFlags to have the
correct number of spaces there

Change-Id: If69de05c4edec4e84c3e059090b9cb89845e3269