Remove the onboard "mailKey" from Paste
Summary:
See T13065 and rP1e2bc7775bc4.
Intended to cover the gap left by D25899.
The actual amount of impacted installations should
be 0, but the migration makes sure all pastes have
a mail key regardless.
Test Plan:
* Create a paste
* Check `SELECT * FROM phabricator_paste.paste;` and `SELECT * FROM phabricator_metamta.metamta_mailproperties;` on the database
* Run `./bin/storage upgrade`
* Check that the mailKeys have been moved to `mailProperties` in the `phabricator_metamta.metamta_mailproperties` table and the `mailKey` column is gone from the `phabricator_paste.paste` table
Reviewers: O1 Blessed Committers, aklapper
Reviewed By: O1 Blessed Committers, aklapper
Subscribers: aklapper, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D25900