Opt-in mentions for general translation editors for each locale

[petya]

As a follow up of #1242, let's have all General Translation Editors for a locale oped-in to receive notification for the hashtag mentions of the locale.

[ocean90]

#2728 was marked as a duplicate.

[ocean90]

#5135 was marked as a duplicate.

[dd32]

In r15895-dotorg

Notifications: Force Translation GTE's to have a locale matcher.

Fixes https://meta.trac.wordpress.org/ticket/1274.

Anyone who has an existing matcher will see two matchers in their notification settings, but they won't get duplicate notifications.

[tobifjellner]

All good, except the wrong tags were used in the change.
We're using "WP Locale" as listed in ​https://make.wordpress.org/polyglots/teams/
(It looks as if the currently applied fix uses the Glotpress locale, which in many cases is different, and even may be ambigous out-of-context.)

[dd32]

All good, except the wrong tags were used in the change.

Oops, I hadn't realised GlotPress was using a different locale slug pattern.

r15900-dotorg

Notifications: Switch to using the WP_Locale rather than GlotPress locale for notifications.

Props tobifjellner.
Fixes https://meta.trac.wordpress.org/ticket/1274