Remove the user identifier from all the user URLs/views

At !113 (merged), creation and login with identifiers was changed to be submitted using POST requests, so that the identifiers aren't part of the URL.

I realized now that all the other views (eg: list projects, issues, etc.) still show the user identifier in the URL.

iirc how Django authentication works, this could be solved in several ways:

• getting user (in this case user_identifier) from the request object
• using a session
• using a middleware

Maybe the UserIdentifier class should inherit from AbstractBaseUser to don't have to implement methods.

Also all the urls that contain <str:user_identifier> should not require the user identifier string.

@whycurliness: is this something you would like to modify? If not, no worries.