allow for usernames with dots

preparing for #5664 with some test improvements i ran into this issue

This commit includes a fix and the test improvements. In particular it
adds BrowserIntegrationTest#login - so there is no need to go through the signup procedure everytime you want a user to be logged in.
keep empty email field if user removed prefill

We should respect the users choice. We can still get their email from the user id if we really need to.
minor: use %Q for interpolated string with "
prefill ticket form from the model - fixes #5657

email and regarding user fields can be set to defaults based on
created_by user.

If these fields are emptied by the submitting user they will be set to  whereas they are nil if they have not been initialized. In that case we will use meaningful defaults from the user who created the ticket.
destinguish user.email from user.email_address

use the former if you want a working email account or nil, the
latter if you want the email address associated with a given
user no matter if the user actually has an email account or not.
adjust tests to new config and method implementation

Ticket.is_creator_vlidated? now actually fetches the user from the db and returns false if it does not exist.
Merge pull request #157 from azul/bugfix/5664-stop-email-autofill

Bugfix/5664 stop email autofill