Functionality to reset statistics

[stklcode]

Background

The question came up quite a few times in various support requests: How can I delete/reset the statistics?

The answer is always the same, direct DB manipulation, setting the storage period to 1 day and back or reinstallation. None of these solutions is complex, but none is really convenient either.

References (selection):

• https://wordpress.org/support/topic/statistiken-zurucksetzen/ (German)
• https://wordpress.org/support/topic/request-delete-option/
• https://wordpress.org/support/topic/reset-stats-5/

Proposal

We should introduce a reset button on the settings page.

The button should be little separated from the save-button, probably red colored with something like "danger zone" above.
A user who can edit the settings is able to reset the data using the ways above anyway, so I don't think we need any additional permissions here.

The function should remove all statistics data from the database and probably reset the tables (e.g. auto increments), so we can start off from scratch afterwards.