MDLSITE-2009 Various 'major' fixes and improvements

* Fix: Allow bumping from any release to 'major'
    (previously it was impossible to bump if the $release had a '+').
* Fix: Allow to execute a 'major' build multiple times. The newly
    created MOODLE_XX_STABLE branch will be reset properly.
* Fix: Solve a problem calculating the name of the new stable branch to be created.
* New: Add a new (internal) type, called 'back-to-dev':
  ** Use it to move master back to dev after 'major' release.
  ** Added support to change $version comments.
  ** Move code previously under 'weekly' to the new type.
* Improvement: Add some notes about to wait for CI servers
    before sending the tags to integration.git