Fix #271

Add back the `GET /api/v1/trends` API with the caveat that it does
not return tags that have not been allowed to trend by the staff.

When a hashtag begins to trend (internally) and that hashtag has
not been previously reviewed by the staff, the staff is notified.

The new admin UI for hashtags allows filtering hashtags by where
they are used (e.g. in the profile directory), whether they have
been reviewed or are pending reviewal, they show by how many people
the hashtag is used in the directory, how many people used it
today, how many statuses with it have been created today, and it
allows fixing the name of the hashtag to make it more readable.

The disallowed hashtags feature has been reworked. It is now
controlled from the admin UI for hashtags instead of from
the file `config/settings.yml`

Fix #6856
Fix #6951

- premailer gem to turn CSS into inline styles automatically
- rework UserMailer templates
- reword UserMailer templates