Allow “@” in user names in URLs.

Sentry Issue: ACE4-1W

NoReverseMatch: Reverse for 'profile-view' with keyword arguments '{'username': 'hedleyd2002@yahoo.co.uk'}' not found. 1 pattern(s) tried: ['profile/(?P<username>[-a-zA-Z0-9_]+)/\\Z']
(44 additional frame(s) were not displayed)
...
  File "ace4/db/templatetags/db_extras.py", line 42, in render
    self.nodelist.render(context)
  File "ace4/db/templatetags/db_extras.py", line 106, in render
    tab['content'] = self.nodelist.render(context)

Django officially allows @ and + in user names now, presumably so it is easier to use e-mail addresses as user names. This means that it is no longer possible to use <slug:username> in URLs, because slugs cannot contain @ or +.