Fix profile tests' warnings if postmap binary not installed

This PR improves the developer experience when running tests (especially profile tests).

Currently, `python manage.py test profile` has a number of passing tests that output messages to the console. These messages ("`/etc/postfix/virtual_alias_maps` not found") are due to the postmap binary not being installed on the local test system (see Advanced Installation docs).

I've commented out a few lines that were flagged to be fixed in `__init__.py` and skipped the test for the binary in the `profile/tests.py` file.

It would be good to fix this more elegantly and perhaps refactor the very long test file after the Django upgrade is completed.