Is your feature request related to a problem? Please describe.
Currently, the numbers bar at the top of the stats page holds 7 sections, where the 5th and the 6th are showing the same information just with other units.

Describe the solution you'd like
I propose to merge those two sections into one, giving the opportunity to switch between the different units with corresponding buttons below the number. This has the following advantages:

• every number section shows a different metric again
• a free slot for a possible other/new metric
• more than 4 units are now possible, e.g. also adding the number of mails per quarter

Describe alternatives you've considered
None.

Additional context
The possible new metric taking the free slot could be starred mails, see #323 (reply in thread)