Add ability to edit texts (only changing title).

Editing text contents after they've been submitted makes no sense, since
it would invalidate all questions, answers and comments based on the "old"
version of the text. So if you want to submit a "new" version of a text,
submit a new text with a new name. However, we do allow renaming texts, so
that the text can be renamed to "text (old version)" or whatever.

This commit has a minor flaw in that it makes another round-trip to the
server to fetch the text title. That should be in the page DTO, but this
will work for now.