[MR Analytics] Add MTTM container (FE)

Problem

Mean time to merge (MTTM) will be a metric our users will want to actively track and manage.

Proposal

• For the time period and filtered criteria, show the mean time to merge (the average amount of time between MR opened and MR merged).

Note: This is the frontend implementation issue. The backend issue is here: #246820 (closed).

Note 2: On the screenshot we see the possibility to choose the time period, but that is not yet implemented, the default time is past 12 months.