Generate pinned and minimum requirements

For testing and deployment purposes, it is very helpful to have pinned
requirements (that won't change when a new release comes out).  It is
also useful to be able to test the purported minimum versions that are
supported, to see if that is in fact true.

This commit does not set up that testing infrastructure, but does add
minimum versions to many requirements, and generates requirements.txt
and a new min-required.txt with minimum claimed supported versions.

This is part of the resolution for Bug 3005.
Generate HTTP Expires headers

Per Django tuning suggestions in bug 2935, this sets image expirations
to 2 hours, and (versioned) CSS/Javascript to 10 years.
Fix broken template formatting

$()s will not be substituted - only %()s
Improve error message

I am constantly setting PYTHONPATH=`pwd` but it has no effect - it needs
to be exported into the environment, make the error message reflect that.
Docs: improvements on upgrade docs
Fix group permissions for REMOTE_PODIRECTORY

also be a bit more careful about these operations in case REMOTE_PODIRECTORY
were not set, you don't want `rm -rf /*`!
Fix typo in MySQL-python rev.spec.
Add public cache control for proxies

This change (also for bug 2935) allows proxies to do caching, as well as
browsers.
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 purge_stage command

Necessary for easily removing deployments performed using the stage_feature
command.
Fix mysql root escalation

The --defaults-group-suffix=_root option has to precede any options
that take separate arguments (like "-u root").
This is true for mysql and mysqldump.
Also, add root escalation for dump_db and load_db commands.
Note exception to MySQL db root escalation
Don't fail fast for purge_stage
Never put sudo and rm -r together