As a **user**,
**I want** some help to pick templates which searching,
**so that** when I search, I get valid results
Acceptance Criteria:
* The UI Widget where I select templates shows a list of available templates when it has focus and i started to type the first letter.
* The UI Widget shows only the templates that match the text that is already typed.
* When a template from the list is selected, it becomes a "pill" inside the UI widget.
Background:
This needs to be implemented as a custom widget. Since `MenuTagMultiselectWidget` expects a fixed number of options and loading ALL the templates into memory would cost too much memory and bandwidth, it's better to subclass `TagMultiselectWidget` and use the `LookupElement` mixin to provide the menu.
The API endoint is `opensearch`. An example query for the German Wikipedia for all templates beginning with "Burg" looks like this:
https://de.wikipedia.org/w/api.php?action=opensearch&format=json&formatversion=2&search=Burg&namespace=10&limit=100&suggest=true