We've decided to add a 'brand' field to help clients decide how to style the
buttons.

Also, fix up the allowed characters for idp_id, while I'm in the area.

This is the final step for supporting multiple OIDC providers concurrently.

First of all, we reorganise the config so that you can specify a list of OIDC providers, instead of a single one. Before:

    oidc_config:
       enabled: true
       issuer: "https://oidc_provider"
       # etc

After:

    oidc_providers:
     - idp_id: prov1
       issuer: "https://oidc_provider"

     - idp_id: prov2
       issuer: "https://another_oidc_provider"

The old format is still grandfathered in.

With that done, it's then simply a matter of having OidcHandler instantiate a new OidcProvider for each configured provider.

This PR adds the missing user_mapping_provider section in oidc.md

Signed-off-by: Christopher Rücker <chris-ruecker@protonmail.com>

Signed-off-by: Peter Krantz <peter.krantz@gmail.com>

This allows for connecting to certain IdPs, e.g. GitLab.

docs, default configs, comments. Nothing very significant.