No prompt to save changes in "Highlights" activity

Test case

Steps

1. Connect to core
2. Go to overflow menu → Core Settings → Highlights
3. Add a new highlight rule or a new highlight ignore rule
   • Specify anything for the name
4. Save the new highlight/highlight ignore rule
5. Press "Up" or the "Back" button on the Highlights activity without pressing Save

Expected

Quasseldroid prompts about discarding changes.

Actual

New/changed highlight rules are lost without warning.

Additional

Ignore List properly handles this. Similar to the issue for IRC servers in Networks.

Marking as Priority: Normal as this is a data loss bug, but for an infrequently changed section.