Make user.optional_attributes a TEXT column

There is no reason to have a fixed limit (as inherent to a VARCHAR column) on that column.
Additionally, the column being a VARCHAR without an explicit size limit breaks on MySQL (fixes hhucn/adhocracy.hhu_theme#428).
The migration script corrects installations that are already on 68/VARCHAR.
Remove useless import
Merge pull request #514 from phihag/fix-optional-attributes-text

Make user.optional_attributes a TEXT column