Enhancement: Option to enable Vue history mode

We've added the option to use vue's history mode. All configuration is done automatically by the system.
To enable it, add a `<base href="PATH">` header tag to `index.html`, `oidc-callback.html` and `oidc-silent-redirect.html`.
Adding `<base>` is not needed for ocis.

https://github.com/owncloud/web/issues/6363
https://github.com/owncloud/web/issues/6277
