Log translation warnings to slack & review existing warnings for usefulness

[dd32]

As discussed during the #meta-i18n chats, we're going to log translation warnings to Slack to make it easier to identify potentially bad translations.

In addition, we need to review what currently creates a warning, and if anything else should do so.

Current warnings include:

• String Length - Translation must be between 20% and 500% of the original string length
• HTML Tags - Translation must contain the same HTML tags & attribute names (The title tag may contain a different translation)
• Links must match (This is built into the HTML Tags checks above)
• Placeholders must match (ie. %s, %d, etc)
• Translation must start/end on a new line if the original does

[dd32]

In 1869:

Translate: Log translation warnings into Slack #polyglots-warnings for peer-review.
This is also likely to help us tweak warning rules to minimise false-positives.

See #1229

[dd32]

In 1870:

Translate: Don't log a warning twice in a single request. This can occur when a translation editor triggers a warning as we both save and approve the string in one move.

See #1229

[dd32]

In 1871:

Translate: Only log translation warnings for translations who are 'current' or 'waiting'. This avoids rejected strings (upon update) being sent.

See #1229

[dd32]

Marking as resolved. Other tickets have added a client-side JS validation & URL validation