Introduce new prefix character for web public streams

[preetmishra]

Currently, both the web public streams and the public streams use '#' as prefix_character which makes it hard to portray the distinction. To resolve this, we could either use 'W' (considering how 'P' is used for private streams) or some other character for the web public streams.

Screenshot

Web app	Zulip Terminal

[preetmishra]

This not critically important for now but would be useful to have post zulip/zulip#14638.

[neiljp]

This is covered in more detail by the broader but duplicate #1135.

[neiljp]

This was fixed by #1177.