Implement Due Dates admin dashboard

[julen]

In order to view all due date rules at a glance, and to manage (add/change/remove) them, a separate Admin tab is introduced:

Here, “Add Due Date” button is disabled until the due date is selected and the path is set. Path can only be picked from the drop-down list (with lookup/autocompletion); paths that already have the due date are listed but can’t be picked.

Existing due date rules are displayed in the table below, along with the progress bar next to each pootle path. Pootle paths link to corresponding browse pages. When the path is 100% translated, due dates are rendered in green color to visually separate completed tasks from incomplete ones.

Prerequisites

• Implement Due Dates widget in browsing pages (Add Due Dates widget in browsing pages #4801)
• Reuse components:
  • TimeSince
  • TranslationProgress
  • DueDate
  • DueDatePicker
• Implement reusable components:
  • Table (with sorting)