Add filters by milestone, author, and assignee to task by type

Resolves #360584 (closed)

The task by type internal API endpoints, used to produce the "Type of work" chart, can now filter by author, by assignees, and by milestone. This makes it consistent with the other charts on the same page.

Using the same convention as the other charts, the filters are specified with the following URL parameters:

• milestone_title - a string with the milestone title to filter for
• author_username - the username of the author to filter for
• assignee_username - it accepts a single username or multiple usernames (like assignee_username[]=username1&assignee_username[]=username2) of assignees to filter for.

This is preparation for frontend change, therefore no documentation changes were made yet.

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Related to #360584 (closed)