Add separate log action for file reverts

Add separate log action for file reverts

This change s adds 'revert' as a separate log action for file reverts, as it
allows special formatting of log entries and revisions in UI and filtering
for file reverts specifically.

Even though there are no log entries with this log action right now, it does
seem that this was intended as some point, as there are corresponding
test cases in UploadLogFormatterTest, and is listed in
$wgLogActionsHandlers and https://www.mediawiki.org/wiki/API:Logevents
as well. Furthermore, the i18n message 'logentry-upload-revert' already
existed before this change.

Because this functionality can not be provided by tags, the 'mw-undo' tag
is not suited for this use case. However, it could be added additionally to
all log entries with this log action..

Bug: T60209
Change-Id: Ie1ccd8053dc5de58b2297a8460219f0233aab968