Issue title: Coalesce proximate notifications from the same source
 Issue body: Getting one email for each location for the covid vaccine is not very nice.
 Comments: 
 Comment 0: Never mind, we only show one location at a time.
{'source': 'GithubIssues', 'repo_name_with_owner': 'stanford-oval/genie-toolkit', 'issue_id': 862051157, 'repo_stars': 153.0}