Add native back buttons to richtext preview

[mpivchev]

Since we use back button from web during rich text preview, If a document cannot load, a user would be stuck on this screen with no way to go back.

We should add a native back button.

[mpivchev]

cc @nextcloud/designers. From discussion with @jancborchardt the plan is to remove the web view back button from NC Text, and eventually from Office. The native back buttons will be used instead.