a) Opening `ComposeSubchannelModal`
1. Open thread menu
2. Click on `Create new subchannel`
`ComposeSubchannelModal` appears.
b) Using form:
First step - `SubchannelSettings`:
1. Enter a new subchannel name. Until the textbox is empty, you couldn't go to next step
2. Choose channel type. By default, it is set to `COMMUNITY_OPEN_SUBTHREAD`
Second step - `SubchannelMembers`:
1. Choose at least one member
2. Click `Create`:
3. `Create` button shows loading indicator:
- if success -> form disappear and currently active thread changes to the newly created subchannel.
- else -> enable `Create` button and show error message
Go back to previous page/step:
1. When you are on `SubchannelMembers`, click on `Back` button
2. `Stepper` changes current active step to `settings`.