create autocomplete component that is triggered by user tag

[DarrellRoberts]

Description

Create autocomplete component that is triggered by the @ character.

Proposed solution

This will work in the same way as the searchbar on the /dashboard/volunteer route with a debounced request and can use a Selector component to contain the results (I imagined you might have to create your own rather than using our current one).

[nadavosa]

Sub-issue of #440 (tagging in comments).