Improved admin tool for adding members to publisher groups

Currently, to add members to a publisher group, we go to https://hypothes.is/admin/groups/new and:

- At the bottom of the page, click "Add Member"
- Enter the Hypothesis username of the member
- Click "Add Member" again to add the next member ... etc

It gets a bit onerous when you're adding a lot of members to a group:

It would be amazing to enter all member usernames without having to click the "Add Member" button in between.

If we plan to make group creation + management self-serve in the future, this would be good to have, but it would also save me a lot of time as a Hypothesis admin.

Start at the admin group creation page at https://hypothes.is/admin/groups/new and inspect the current member-entry flow. Done means an administrator can enter multiple member usernames without clicking “Add Member” between entries, while preserving the existing group-creation behavior.

Written by the indexing model from the issue text.