[feature] Autocomplete for watch label / tag from existing values

**Version and OS**
web

When adding a new change detection watch, need to manually write the whole watch label / tag.

**Suggested Solution**
Autocomplete for watch label / tag from existing values

**Example use-cases**
- Initially adding multiple change detection watches with some labels/tags.
- Adding a new change detection watch.

**Additional context**
1:

2:

Existing watch label / tags:

The issue does not identify files, tests, or an entry point. Locate the form for adding a change detection watch and how existing watch labels or tags are loaded; the work is done when users can choose or complete labels/tags from existing values while adding a watch.

Written by the indexing model from the issue text.