Policy email alerts

WIP so @equivalentideas and I can work out what we've got left to do.

Fixes #939.

TODO:

- [x] get links in the emails working
  - [x] policy name
  - [x] division added/removed/updated name
  - [x] wikimotion division name
  - [x] attribution user name 
- [x] Alert when a Division is edited
- [x] Check content and style emails
- [x] Fix email subject/from address
- [x] style hover states of the subscribe button
- [x] Add unsubscribe to email footer
- [x] Add list of things you are watching
- [x] fix plain paragraph tags being inserted into the text version
- [x] Corner case: a division is connected to two policies and you're watching both of them (surely you don't want two emails in this case) [WONTFIX: We're going to see how this plays out first - maybe we do want two emails?]
- [ ] Ensure time zone is correct and add time zone
- [ ] add tick to subscribed button