Add Matomo integration

[nichtich]

Optionally track usage with Matomo. To do so add configuration key matomo with value object passed to vue-matomo configuration.

First implement without consent. Then add a setting in the "My Data" tab of TheSettings component (see methods of this.$matomo documented at https://developer.matomo.org/api-reference/tracking-javascript) so users can enable/disable tracking. For first use, we also need to show an alert, asking user to enable tracking.