Quoted response removal by fuzzy hash

This feature removes the necessity of the "QR tag". When mail is generated, a fuzzy hash of the text and html pieces are saved in the database. When mail is received, the hash of the received mail is compared with the hashes on record. The one with the closest match is used. Then, the parts of the message which match the hash on record are removed, leaving the email response remaining.

### Outstanding
  * [ ] Add migration script
  * [ ] Support old-style QR removal (for lingering e-ails after upgrade
  * [ ] Handle links being linkified (by mail clients) in HTML
  * [ ] Handle `<img/>` tags being replaced by text
```sql
  `hashes` text CHARACTER SET ascii COLLATE ascii_bin,
```