[web] Introduce ComposeSubchannel modal

Summary:
Depends on D5186
Context: 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

• SubchannelMemebers, which contains potential members list, last step

Test Plan:
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.

Reviewers: tomek, atul, ginsu, rohan, abosh

Reviewed By: tomek, atul

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5187