Synchronize the discard of translations warnings between "dev" and "stable"

[tobifjellner]

If you as an editor on translate.wordpress.org translate either "dev" or "stable" of for a plugin, the translations are automatically copied between "dev" and "stable" when you enter them.
But: If there's a warning (different URL, missing tag, big difference in string length, etc.) you'll have to "discard" a warning before the translation is entered.
Now: If you discard the warning, this string will still be added as "waiting with warning" in the sister project: in case of warnings, you currently need to visit the sister project and discard the warnings there, too.

[garrett-eclipse]

Original discussion found here;
​https://wordpress.slack.com/archives/C02RP4R9F/p1537551803000100

The dismissal of a GP warning triggers the gp_warning_discarded action which can be hooked into similarly to the other sync events ‘gp_original_created’, ‘gp_translation_created’, ‘gp_translation_saved’;
​https://github.com/GlotPress/gp-translation-propagation/blob/master/includes/class-gp-translation-propagation.php#L42-L45

As long as the strings match between the dev/stable projects it should be ok for the warning discard to propagate.

[ocean90]

In 11041:

Translate: When syncing plugin translations ensure the current status is also copied.

See #3828.

[ocean90]

In 11068:

Translate, Plugin Directory: During translation sync also discard warnings for existing translations.

Fixes #3828.