Refactor and cleanup editor dialogs

Collection task:

Refactor id vs. class in editor dialogs (which are now overlaying each other ->duplicate id problem)
Improve selectors in js code, which (1) need to adress classes and (2) should only work on own modal

(from redmine: issue id 547, created on 2018-01-22tgradl, closed on 2018-01-22)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information