AuthCheck: docstring, garden
Add require_valid_email attribute to instance

This attribute determines whether email validation is mandatory in this
instance.
Allow check for validated email (fixes #142)

Add valid_email check to AuthCheck and use it in all instance related
permission checks for write (not readonly) actions.
Add attention getter for missing email validation
Allow dynamic overlay feature for actions needing valid emails
Use dynamic overlay for proposal create button

The same should be done in all other places.