[web] Introduce CommunityCreationKeyserverLabel

Summary:
Pulled common keyserver label out of CommunityCreationModal and CommunityCreationMembersModal.

Why not make it more general and put it in components?

It has a hardcoded keyserver name (ashoat) so I think it makes sense to keep its use as limited as possible. CommunityCreationKeyserverLabel could be the basis for a more general purpose KeyserverLabel component in the future.

Test Plan: Things continue to look as expected w/ CommunityCreationModal and CommunityCreationMembersModal.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7894