Add an add another autocomplete to the Git attribution info / commit messages on Drupal.org

Problem/Motivation

Follow-up to #2295411: Auto-generate Git attribution info / commit messages on Drupal.org

There are cases where a committer needs to credit users who have not commented on an issue. There is no way to get this info into the database that #2295411: Auto-generate Git attribution info / commit messages on Drupal.org established. Committers can still manually add users to the commit mentions though.

If we want to move away from parsing the commit mentions, then we need to add a way for committers to add arbitrary people to the db record.

Use cases:
1) multiple issues have work on them, but are duplicate. Then the committer should add users from the one closed as duplicate to the issue which has the commit on it.
2) work is done outside the issue, for example, a designer working on a style guide, and then a particular issue implementing that. (came up during dev days france)

Proposed resolution

Add an "add another" autocomplete where a committer can credit people who have not yet commented on an issue.

Remaining tasks

User interface changes

API changes