Add component filtering based on key about weblate

[nijel]

Just found out #8084, but closing that in favor of this as the description is way better here.

PS: #1993 might also be related.

[github-actions]

This issue seems to be a good fit for newbie contributors. You are welcome to contribute to Weblate! Don't hesitate to ask any questions you would have while implementing this.

You can learn about how to get started in our contributors documentation.

[nijel]

To implement this:

• Add Key filter field to the Component model
  • Add validation to force it empty for bilingual files which do not use keys
  • Include it in component creation forms
  • Include it in settings form
  • Include it in the API serialization
  • Add documentation for the field with some examples for the filters
• Implement actual filtering in Translation.check_sync
• Add tests to see the filtering is applied
  • Initial creation of a component with a filter should filter out strings
  • Adding a filter to an existing component should remove filtered out strings
  • Removing a filter from existing component should bring back filtered out strings