stream_creation_form: Remove `optional` from stream description input.
stream_creation_form: Remove `optional` from stream description input.

The idea behind this change is to encourage users to enter a stream
description rather than take the easy way out and leave the 'optional'
field empty. This solution intends to solve the same issue as #15997
but in a more indirect way.

Discussion in this thread:
https://chat.zulip.org/#narrow/stream/2-general/topic/Stream.20description.20mandatory.20setting.20.2315997

zulip/zulip,brainwane/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,brainwane/zulip,rht/zulip,zulip/zulip,brainwane/zulip,rht/zulip,kou/zulip,andersk/zulip,kou/zulip,brainwane/zulip,showell/zulip,brainwane/zulip,showell/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,showell/zulip,kou/zulip,showell/zulip,eeshangarg/zulip,hackerkid/zulip,showell/zulip,zulip/zulip,hackerkid/zulip,showell/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,kou/zulip,rht/zulip,rht/zulip,punchagan/zulip,brainwane/zulip,punchagan/zulip,andersk/zulip,kou/zulip,showell/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,brainwane/zulip,kou/zulip