fix broken/missing l10n in devhub

[eviljeff]

Fixes: mozilla/addons#15974

Description

Random fixes for broken/missing l10n in devhub.

Context

The description tooltip was broken because we had a conditional in the middle of the string (it needs to be repeated instead); highcharts dateFormat function wasn't localized, but we can use the javascript built-in instead.

Testing

Mostly just a matter of going to the correct page in a non-English locale

• devhub upload details step for description
• devhub edit details for description
• devhub edit details for contribution url
• devhub upload first step for theme upload blurb
• stats dashboard, and choosing any of the views that have a data table

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.