fix page/proposal title validation

[xi]

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).

[nidico]

We've discussed that in length and came to the conclusion that this PR makes things better, even if the behavior is still a bit bizarre (you can create duplicate titles only by renaming) and we still don't fully understand whether labels have a meaning in proposals. However there is at least a clear concept (labels are unique and never change whereas titles can be changed).

We also discussed whether we need a migration due to this: Due to the previous behavior, there can be delegateables with duplicate labels, which we now assume to not exist. This shouldn't matter much though (or not less than before).