Bug 1072921: Add more isolation of allauth-related tests.

Two things going on here:

1. Expand the work done in 183fa261c7e7a8cce216b3f4977154a787ba124e to
   the test cases in test_templates.py. Although there's less going on
   in that file and so probably a lower risk of order dependence, we
   probably want more tests there later and so it's a good bit of
   future-proofing.

2. When a unique email address and/or username is needed for test
   isolation, instead of using sequential numbers use a name/address
   based on the name of the test method in which it's used. This will
   hopefully avoid issues down the road where we might lose track of
   which numbers had already been used in existing tests.