Add support for multiple Proton accounts

[bartcode]

Whereas Proton Mail does support having multiple accounts, Calendar does not. Would be good to have that implemented for people who have a private and business Proton account for instance.

[juxuanu]

Insane this can't be done. Truly discouraging

[bartcode]

Since Proton ignores the issues in this repo, I ended up sharing my business calendar with my private account.