Fixes #91

This PR fixes the following issues with single-room mode:

1. In single-room mode, user cannot access the settings #91
2. In non-small screens, single-room mode was not functional, since user still had access to list of rooms.
3. User could not access session picker when in single-room mode.

This PR implements the following changes, which fix all above issues.

When in single room mode:

1. Hide the left panel completely
2. Add a Settings entry to the options menu of the room
3. Closing the Settings screen navigates to configured roomId
4. Closing the Room (timeline) screen navigates to session picker

Screen captures

Small screen

Non-small screen