feat: Notify translators about invalid entries

[OmarIthawi]

Description

Translators can be notified about invalid translations via multiple channels about invalid entries that has
been found by the validate-translation-files.yml workflow:

• Create a Transifex issue only: It would be possible to identify the
  faulty entries and create a Transifex issue via API to the faulty entries.
  However, this would require attention from the translators
  team, which can take a rather long time therefore creating a backlog of
  invalid entries. Additionally, Transifex issues are not understood or used
  by most of the Open edX community.

• Post errors on Slack only: Post the errors on Slack and ask the
  translators to fix them.
  Same as the previous option, not all translators are on Slack.
  Additionally, this option would have a slow feedback loop causing the pull
  requests backlog to build-up.

• Mark faulty entries as unreviewed only: Use the Transifex API to mark
  the the invalid entries as unreviewed, then pull only reviewed entries into this repository.
  This option requires an extensive use the of the Transifex API,
  and pull again which can be complex to implement. Additionally, this option
  would have a slow feedback loop causing the pull requests backlog to
  build-up.

[brian-smith-tcril]

link to transifex page that lists reviewers for languages

generic: https://app.transifex.com/open-edx/teams/
openedx-translations project, fr_CA: https://app.transifex.com/open-edx/teams/147691/fr_CA/

[OmarIthawi]

Translations Working Group agreed to post the errors into Slack without automated tagging as an initial version of the notification.