[WIP] Add comments mailer

Alert library owners about new comments on their libraries.

TODO:
- [ ] A button in the user profile to turn off the notifications.
- [ ] Test if emails get "sent"
- [ ] Test if emails only get sent if the library owner subscribed to it
Line is too long. [88/80]<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.