Save modified guest in migration.

Part of issue #220.
Add configurable sort order for taps.

Primitive admin interface (form) right now, good opportunity
for some drag-and-drop JS magic.

Part of issue #223.
v0.9.18
twitter: Handle download errors; add picture settings.
twitter: Use the new name for picture settings.
Add static files to troubleshooting.
docs: Fix setup-kegbot illustration.