stream settings: Extract enable_subscriber_management.
stream settings: Extract enable_subscriber_management.

The name here is accounting for future plans where
we will share code for both of these use cases:

    * editing subscribers on current stream (now)
    * editing subscribers on new stream (upcoming)

kou/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,zulip/zulip,kou/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,zulip/zulip,andersk/zulip,kou/zulip,andersk/zulip,kou/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,rht/zulip,kou/zulip,eeshangarg/zulip