browser: implement share by mail

[alexanderdd]

Is your feature request related to a problem? Please describe.
You want to share an e2ee folder with someone, you have their email address

Describe the solution you'd like

1. In the sharing sidebar of an e2ee folder, type an email address of an external user (who has no account on this instance)
2. it should work, like with regular folders. It should send two emails, one with the link, and one with the e2ee mnemonic. External user can then access the files in the folder.

Describe alternatives you've considered
You can already create public links, and send link and mnemonic to external users, but this needs many extra steps