[Export] Daily summary

[micz]

Export a daily summary from the Today and Yesterday Tab.

From #74 (comment):

example csv:
email account; emails send; emails received; send to internal; send to external; received from internal; received from external
test@example.com; 20; 25; 15; 5; 15; 10

[micz]

Related to #335.

This is a big implementation, because for all accounts the stats are calculated without the account info.
Removed the 2.1 milestone, it won't be present in the first release.