Can't add a server to "Select a room directory" when adding a room

#### Describe the bug
Missing option to choose a user defined server for exploring room directory.

#### To Reproduce
1. Go to the rooms screen.
2. Click on #+ to add a new room.
3. Click on ... to change network.
4. Observe how you cannot add another server there apart from the pre-canned options. The options are:
- my.homeserver.org
- All native Matrix rooms (what in the world does this mean!?)
- All rooms on matrix.org server

#### Expected behavior
Option to add another homeserver domain in which to browse the room directory.

#### Additional context
- App version: 1.1.7, Google Play Store