Use buildscripts for CH deployment and update

[aaaaalbert]

The clearinghouse codebase still uses a separate CH deployment/update script. We should retire this script, and move to a workflow that relies solely on the Seattle buildscripts and Git. The high-level plan would be something like this:

When you set up the CH, you'd

• Install and configure the dependencies like Apache, MySQL/MariaDB, and Django,
• Check out the CH repo,
• Configure it (website/settings.py etc.),
• Possibly commit your config locally,
• Possibly push it to a secure backup location,
• Run the buildscripts to
  • check out other Seattle dependencies and
  • deploy the CH.

Now when the CH or its Seattle dependencies are updated (see also #150),

• Return to the directory where you initially checked out the CH repo,
• Remove the DEPENDENCIES dir,
• git pull whatever changes to the codebase; merge them as required,
• Re-run the build scripts to deploy the updated CH into a fresh directory,
• Finally replace the old deployment dir with the new one.

AFAICT, this should be a clean buildscript compatible way to update a deployed clearinghouse. Comments welcome!