Add gp-translation-helpers for the Reject with Feedback Feature

[akirk]

In order to introduce the ​Reject with Feedback to translate.wordpress.org, we'll add the ​GlotPress/gp-translation-helpers plugin to the meta repository.

Development mostly happens in the ​git repository, we'll sync it to svn on a "nightly" basis.

The feature and plugin is being regularly discussed in the ​weekly polyglots meetings.

cc @amieiro @spiraltee

[akirk]

In 11837:

Translate: Introduce the gp-translation-helpers plugin

See #6310, ​https://github.com/GlotPress/gp-translation-helpers/
props amieiro, spiraltee