Multi merchant support

[brianweet]

While the library has multisite support (by configuring multiple redirect urls), there's no support for multiple merchants using different Vipps Login apps.
This is a likely scenario on Epi when different sites want to maintain their own branding/naming.
Multiple merchant applications will be configured in Vipps, and it should be possible to configure Vipps login multiple times in Epi.
The 'sub' claim is only unique per merchant, therefore it is not a real unique identifier but only a unique identifier for a specific merchant.