Display current frequency in unsubscribe link

When clicked through by email display the current frequency in the
frequency selector.
Add opengraph and twitter card to digests
Fix bug in subscribe when frequency was not set
Better error handling

Fixes an issue where if one stream update, digest generation, or email
send failed the program aborted. Now we can continue processing other
items on failure.
Add OpenGraph, Twitter, Facebook support

Use OpenGraph for displaying digests on Facebook and Twitter.