This commit refactors the 'get_subscribed_streams_for_user' function
to return an object with two keys: one for the subscribed streams
and another for the streams the user can subscribe to. The node
tests have been updated accordingly.
Renamed 'get_subscribed_streams_for_user' to 'get_streams_for_user'.
This should cause no functional changes.
This is part of a multi-step effort to move away
from using stream names to reference streams, now
that it's impossible for a user to write a message
with an invalid stream name (since switching to
the dropdown).
We should just accept `subscribed` and `previously_subscribed` fields as
arguments to this function to avoid adding them on the `attrs` object
itself to simplify the type for `ApiGenericStreamSubscription`.
We should always return `boolean` type instead of `boolean | undefined`
type for functions `is_subscribed_by_name`, `is_subscribed`,
`is_web_public` to stay consistent with other similar functions which
have the type of `boolean` like `is_invite_only_by_stream_name` and
`is_web_public_by_stream_name`.
