[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.

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.

Edited Sep 17, 2020 by Magdalena Frankiewicz