bug 1110904 - store revision IPs for superuser review

* [ ] Run `./manage.py migrate` to create the new `wiki_revisionip` table for the new `RevisionIP` model
* [ ] Add a [waffle flag](https://developer-local.allizom.org/admin/waffle/flag/) called `store_revision_ips` with Everyone set to 'Yes'
* [ ] Edit a document
* [ ] As a superuser, go to [the Revision dashboard](https://developer-local.allizom.org/en-US/dashboards/revisions)
* [ ] Click the 'View in admin' link in the 'View IP' column to see the IP address recorded for the revision
* [ ] Change the [waffle flag](https://developer-local.allizom.org/admin/waffle/flag/) called `store_revision_ips` with Everyone set to 'No'
* [ ] As a superuser, verify the "View IP" column is gone from [the Revision dashboard](https://developer-local.allizom.org/en-US/dashboards/revisions)
* [ ] Edit a document
* [ ] As a superuser, verify there's no [Revision Ip](https://developer-local.allizom.org/admin/wiki/revisionip/) record for the edit when the flag was set to 'No'

TODO:

* [x] Create `delete_old_revision_ips` management command to delete all RevisionIP objects older than 30 days
* [ ] File a WebOps bug to add a daily cron job for `delete_old_revision_ips` command
This feels like something that should live in the models file (probably as a manager method) and then just be lightly wrapped by a task.