JQL filter does not autocomplete usernames correctly

Issue Summary

When using the JQL filter for a Slack notification subscription, the autocomplete matcher is not properly filtering usernames. Regardless of the name typed, the same set of users is shown in the dropdown list.

Steps to Reproduce

1. Create a new subscription or modify an existing subscription
2. Switch to JQL filtering
3. Add a username field to the filter - examples:

   reporter =

   or

   assignee =

4. Observe that the dropdown returns alphabetical results (A -> Z) of all users
5. Start typing a username

Expected Results

Dropdown starts filtering to show only matches from the partially-typed user name

Actual Results

Dropdown results are not filtered

Workaround

1. Create a regular issue search in Jira
2. Use the autocomplete filter to select the user you wish to add to the Slack JQL filter
3. Copy the user's ID from the regular issue search - example: 

   assignee = 7b026df108a7a0020d7dcab9

4. Paste the user's ID into the Slack JQL filter