[TASK] Refactor flashmessages.js

Refactor flashmessages.js:
* rename to FlashMessages.js
* match severities from PHP flashMessages
* remove ExtJS
* introduce require.js module
* introduce bootstrap alert message code
* init global one time for complete backend
* load component in top context, referenced now with
  top.TYPO3.Flashmessage.* on any page possible

Resolves: #62893
Releases: master
Change-Id: I10a5222a0fbdf371abcc24d2ebb233a8a9fec150
Reviewed-on: http://review.typo3.org/34029
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>