fix page/proposal title validation

Unfortunately, #849 did not really solve the issue but revealed many more problems. As a result I refactored the whole page/proposal form validation.

I am sorry for the large commit. I did not know how to split this up. Some notes that might be helpful:

-  I made a clear distinction between `title` (of a text), `label` (of a page) and `variant` (not sure what this is)
-  I basically reverted e829af4 because on second thought, page labels should be unique on the instance level (because they are used in URLs) and the description of a proposal *is* a page.
-  While I was sure that labels should be unique, I was not so sure about `title`. I chose to not make this unique. This means that user can not create a page/proposal with an existing `title` (because that would result in a duplicate `label`) but they can later change the `title` (which leaves the `label` as is).