Depends on D5186
Context: [[ https://linear.app/comm/issue/ENG-1000/composesubchannel-on-web | here ]]
Introduce modal for creating subchannels on web.
We would like to allow users to compose community subchannels on the web.
This diff contains complete user interface for creating all 4 allowed types of subchannels (open/secret; standard/announcement).
In this component, we use `Stepper` first time. This modal contains 2 pages/steps:
- `SubchannelSettings`, which is intended for changing subchannel configuration
{F175193}
- `SubchannelMemebers`, which contains potential members list, last step
{F175195}