Fix percentages in usage breakdown having too many digits

Change trending hashtags to only ask for review if a hashtag enters
the top 3 position in the set, since it's the only items shown in
the default web UI

Allows determining where the majority of posts in a hashtag come
from on a given day at a glance.

Add better sorting defaults to the hashtags admin UI

Add "not reviewed" filter to hashtags admin UI

* Fix account tags not being saved correctly

Regression from f371b32

Fix Tag#discoverable not returning tags where listable is nil instead of true

Add notice when saving hashtags in admin UI

Change public hashtag and directory pages to return 404 for forbidden tags

* Remove unused locale string

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 #5578