bug 1069359; init virtualenv on login

Pretty straightforward.  I expect that this will be (eventually) replaced by something more programmatic down the line, but imho, this is a convenient addition with little incurred technical debt. :money_with_wings:

@rhelmer @bramwelt `r?`
This can be simplified to a single test: ``if [ -r "$VENV" ]; then``

``-r FILE - FILE exists and read permission is granted``