Members given access date/time is wrong (request date is shown)

Summary

I requested access to a Group on Friday and got the acces on Monday. In 'Group->Members' it's shown as "Given access 2 days ago" instead of "Given access 5 minutes ago".

The request timestamp is used instead of the granted timestamp.

Steps to reproduce

Request access for a group and wait some time before granting.

Using GitLab Enterprise Edition 11.3.4-ee gitlab-ce@14d3a1dcd514c77fbbd9646d39e3729eecf03b1c

Implementation guide

• Add a new column to the members table called request_accepted_at
• Set request_accepted_at column in app/models/member.rb#L453 when the access request is accepted.
• In the next MR in the next milestone: Backfill that column to created_at when requested_at: nil && invite_token: nil && NOT invite_accepted_at: nil.
• Expose request_accepted_at in app/serializers/member_entity.rb#L12
• In app/assets/javascripts/members/components/table/member_activity.vue#L31 use member.requestAcceptedAt if it is not null, otherwise use member.createdAt.
• 1 milestone after the backfill, the fallback for member.createdAt can be removed, see Remove `this.member.createdAt` fallback once `r... (#468306 - closed)