Add about section

[juliusknorr]

As proposed by @karlitschek in #57 an about section should be added to the settings menu, to explain Nextcloud and our missing better to users.

I would just show the first run wizard again in case the user clicks that about link or should we add a separate page to give some more detailed information about Nextcloud there?

cc @nextcloud/designers @jospoortvliet

[karlitschek]

@juliushaertl I think it is fine to just show the first run wizard again. All the information has to be in there anyways.

[juliusknorr]

Implemented in #63