Backport "Add UI option to block deleted rooms from being rejoined" PR #12
Description
Please, backport changes from Awesome-Technologies/synapse-admin#166 PR
Please, use the following format of commit message to properly credit the original author and provide a link to the context:
Add UI option to block deleted rooms from being rejoined
This is almost a copy of https://github.com/Awesome-Technologies/synapse-admin/pull/166 PR,
authored by @jkanefendt