[Guests] email addresses with + create different guest accounts #3984

Describe the bug

As a guest user, I may want to filter my incoming mail using the format user+filtertag@example.com (if supported by my mail provider. E.g. gmail supports this.).
As per RFC 5233 (Sieve Subaddress) an address of the form user+tag@example.com is meant to be delivered to the same user@example.com regardless what the +tag is. Mail-relay support for this is optional, though.

Steps to reproduce

1. user@example.com tells opencloud user alan to send the guest invite to user+oc-alan@example.com
2. the guest invite is received and clicking the link works. OK
3. the name of the guest user at opencloud is user+oc-alan@example.com and he does not see shares that were created with a different tag. BAD.

Expected behavior

When creating the guest user, the +tag should be stripped.

Actual behavior

Multiple guest users get created, for the same email user person.

Setup

Seen with opencloudeu/opencloud-rolling:9.1.0

Additional usecase

If a regular opencloud user wants to receive a shared item as a guest, rather than a regular opencloud user.
His email address opencloud-user@example.com cannot be used for that, as the sharing dialog automatically matches this with the known username.
Supporting RFC 5233 would allow e.g. opencloud-user+guest@example.com