[TASK] Cleanup GridEditor.js

- use === for comparrision instead of ==
- build markup with jQuery api instead of string concatenation
- refactor stripMarkup function

Resolves: #75733
Releases: master
Change-Id: I6b32cdd4f8a2e137bdb7e6c9c88efd831ebdf5e3
Reviewed-on: https://review.typo3.org/47742


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christoph Kratz <ckr@rtp.ch>
Tested-by: Christoph Kratz <ckr@rtp.ch>
Reviewed-by: Sascha Egerer <sascha@sascha-egerer.de>
Tested-by: Sascha Egerer <sascha@sascha-egerer.de>