Long usernames overflow when managing watchers

Summary

While managing watchers if the username is long, it overflows from the dropdown box.

How to reproduce

1. Navigate to a page
2. Manage watchers for the page
3. Type a long username in the add watchers list

Expected behavior

Long usernames are truncated.

Actual results

Long usernames spill over the edge of the dropdown.

Workaround

Currently none