User search widget

This adds a barebones user search widget, using the user search API introduced in 1a8deeaf.

Views to be updated to use this:

• Role management - index - search
• Role management - role group - add user
• Case - edit - assign advocate
• Case task - create - task assignee
• Case task - edit - task assignee

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To enable this for a field that accepts a user ID as an integer:

• Add the .user-search-field class to the field element
• Add the async-load stanza to the page to actually load the widget (.async-load{:'data-sources' => JSON.generate(['/static/user_search.bundle.js'])})