app

On notifications page, in settings, "Show" for quick filter bar and
unread notifications markers use the same string, while being placed in
the separate contexts - under different settings categories.

This commit splits the contexts, creating new strings:

- "Show filter bar" in place of "Show" for filter bar;

- "Highlight unread notifications" in place of "Show" for unread
  notification markers, which are now simply called "Highlighting";

- There's also "Unread notifications" in place of "Unread notification
  markers" string to accompany the changes to "Show" string under that
  category.

All new strings have new IDs, which will cause previous strings to
invalidate on the next translation update and new strings to be created.
This will help Mastodon translators to know that there are changes and
improve translation in accordance to the source string update.