Add to the [[webapp]] some configuration of git-annex.

There are some basic settings that pass through to `git config`, things
like how much disk space to leave free, how many copies to ensure are kept
of files, etc.

The meat of the configuration will be in configuration assistants that walk
through setting up common use cases.

* Create a repository (run when the web app is started without a configured
  repository too). **done**
* Clone this repo to a USB drive or other removable drive. **done**
* Make a bare repo on a remote ssh server **done**
* Clone this repo to another host. (Needs [[pairing]]) **done**
* Set up Amazon S3. **done**
* Set up encrypted rsync remote. **done**
* Rsync.net special case **done**
* Set up gpg encryption key; gpg key distribution.
* I lost my USB drive!
* etc -- many more possibilities
