Issue title: PGP and Uppercase
 Issue body: Hello,

When I've generated my PGP key, I put an uppercase at the first letter of my mail address.
Now, I've got a problem with rainloop because it use lowercase and so, my pgp key isn't recognised...
Can rainloop be case insensitive when it check for pgp correspondence ?

 Comments: 
 Comment 0: Thanks

 Comment 1: I think you will have problems if you try to implement this, and I am not sure what RFC to cite, but domain names are case insensitive and usernames are not!  

{'source': 'GithubIssues', 'repo_name_with_owner': 'RainLoop/rainloop-webmail', 'issue_id': 41272538, 'repo_stars': 3783.0}