Better handling of multiple account provider

[CarlSchwan]

In https://flathub.org/login, when login with the github provider and then using any of the gitlab providers, this will create two accounts instead of reusing the first account.

This become an issue because then unless one of the two accounts is deleted, the two login providers can't be linked to the same account with in https://flathub.org/settings

It would be great if the accounts could be merged or at least the error message be a bit more explicit how to resolve the issue (deleting one of the two account).