Fabric: Provide a way to alter Fabric settings

This is necessary for bug 3004.
Fabric: Add stage_feature command for testing feature branches

Now it is possible to deploy a Pootle instance using a specific repo and a
specific branch, and copying a previous Pootle DB to bootstrap the deploy.

Fix bug 3004
Cleanup: use abort() from Fabric
Minor cleanups and fixes in Fabric and docs
Fabric: Add unstage_feature command

Necessary for easily removing deployments performed using the stage_feature
command.
Cleanup: use env_path instead of building the path
Display unit context as well
Docs: improve examples for update_code Fabric command
Change the latest activity message shown inside a TP

Now the message:

* is shown after the folder stats inside a box,
* includes an italiziced snippet from the unit source (or target), if
  applicable,
* the snippet shown might be trimmed to 40 chars, appending an ellipsis if
  trimmed.

Fixes bug 2964
Cleanup: Remove unnecessary import since Pootle supports only Django >= 1.4
Docs: Update instructions for deploying using Apache with mod_wsgi

It might help using a configuration more similar to the ones:

* After installing Pootle using pip
* After installing Pootle using Fabric
Docs: Add WSGI script example for deployment using Apache with mod_wsgi

Fixes bug 2688
Docs: Change WSGI and Apache examples for providing a custom settings file

This allows having Pootle settings customizations in a custom location outside
Pootle code, perhaps along with the WSGI script.
Cleanups
Docs cleanups
Cleanup: Remove hardcoded repo URL in Fabric
Cleanup: Add some prints in Fabric

This will allow to see in the command line some more descriptive messages
when running some commands.
Don’t send emails to every registered user

News emails are sent to every registered user even when sent from a
given project, that is because get_matching_permissions() falls back to
checking the permission of user “default” when the given user has no
explicit permissions for that directory, but this is not desired here.

Fixes bug 3021.
Cleanups: More normalized messages when running Fabric commands
Typos in docs