Import github 'de/milestoned' issue events

What does this MR do and why?

It's an implementation of GitHub Importer - Migrate issue 'de/milestoned' events

There is a nested query:

project.milestones.find_by_title(issue_event.milestone_title)&.id

SQL of this query looks like:

SELECT
    "milestones"."id"
FROM
    "milestones"
WHERE
    "milestones"."project_id" = 33
    AND "milestones"."title" = 'milestone1 long title'
LIMIT 1

Screenshots or screen recordings

Local test

How to set up and validate locally

Create project via import from Github. The project should have the milestone that was assigned and unassigned from an issue at least once. After importing, make sure that the issue has milestoned and demilestoned events with the appropriate author and time.

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.