Since it is footgun-prone, discourages upstreaming of useful development
tricks & is unnecessary in an environment variable centric world.
The one remaining `BZ_API_URL` setting isn't actively used, and if this
changes in the future, it should be set via an environment variable
instead.
Since they're not specific to the Django app 'webapp'.
Whilst we're there, the local & example settings files have been
renamed. In the future I'd like to combine settings_local.example.py
with puppet/files/treeherder/local.vagrant.py, but I'll do that in
another bug.