Fix type icon of PhabricatorMetaMTAApplicationEmailPHIDType
Summary:
`fa-email` does not exist in FontAwesome per http://phorge.localhost/uiexample/view/PHUIIconExample/.
Use existing `fa-envelope` instead like the codebase already does in other email related places.
Test Plan:
Go to http://phorge.localhost/config/module/phid/ and look at the "APPE" line, see an icon now.
(No, I have no idea why four icons use `bluegrey` while the rest does not, sorry.)
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26420