Show muted streams as faded in streams list

[gnprice]

This is a followup to #346. We have all the relevant data handy, so this is just a matter of applying the desired style.

@terpimost do you have a design for this already in Figma? I see the streams screen at https://www.figma.com/file/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=112%3A9610&mode=dev , but it doesn't look like any of those are shown as muted.

[terpimost]

I don't have the design yet. but I suggest to make them 50-60% transparency:

But we do want the background to be white, otherwise it would break the list

[dhruvpatidar359]

may i work on this issue , if the design is decided ?

[gnprice]

Yeah, the draft design above is enough that we can go ahead and implement this. We can always adjust the design later.

@dhruvpatidar359 do take a look though at our README section on picking an issue to work on: https://github.com/zulip/zulip-flutter#picking-an-issue-to-work-on