[web] introduce AutoJoinCommunityHandler to web

Summary:
This diff simply introduces the version of AutoJoinCommunityHandler for web

Depends on D12716

Test Plan: Confirmed that the auto join community mechanism also works on web

Reviewers: ashoat, inka

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12717