[Feature]: Add Matrix handle to profile

[J0WI]

In addition to DECT, E-mail etc. I'd like to add may Matrix handle so that others can contact me over Matrix or recognize me in rooms.

The field should be optional and independent from E-mail and nickname.

[MyIgel]

Having them as "normal" fields will somewhat bloat the config / options.
Eventually adding freely configurable inputs might help but thats a thing for some point in the future when we have a more robust way to extend the engelsystem / add plugins / insert more dynamic content.