Accept maintainer_note parameter in REST runner registration

What does this MR do and why?

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This MR adds support for taking in a maintainer_note field in the runner registration endpoint. A gitlab/gitlab-runner MR will soon take advantage from this addition so as to allow an admin to specify notes to be associated with the new runner.

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Part of #348299 (closed)

app/assets/javascripts/groups/groups_list.js

-import FilterableList from './filterable_list';
+import FilterableList from '~/filterable_list';
 
 /**
  * Makes search request for groups when user types a value in the search input.