Added OIDC Support

[Lachee]

Adds OIDC Support for login

Configurable with the server config:

# OpenID Connect (OIDC) single sign-on
# Set client_id, client_secret, and issuer to enable SSO login.
# redirect_uri must be your site's base URL (the SPA root), registered with
# your OIDC provider exactly as written here.
oidc:
    client_id:       # example: my-client
    client_secret:   # example: mysecret
    issuer:          # example: https://auth.example.com
    # must be registered with your OIDC provider and must point to your site root
    redirect_uri:    # example: https://example.com
    scopes: 'openid email profile'
    # set to no to prevent automatic account creation on first OIDC login
    allow_registration: yes
    # disable username/password login and self-registration; users must log in via SSO
    disable_password_auth: no
    # optional custom label for the SSO button (default: 'Log in with SSO')
    button_label:
    # optional URL to an icon image shown on the SSO button
    button_icon:
    # OIDC claim containing the user's avatar/profile picture URL
    # standard value is 'picture'; leave blank to disable avatar import
    avatar_claim: picture

You can:

• Setup OpenID Connect
• Disable password logins
• Disable new accounts
• Customise login button
• Auto-load a users avatar.

In the future, there could be a potential for roles based on the OIDC claims too, such as with PocketID's groups.

[Lachee]

I will be honest, I didnt mean to make a PR to the main repo, but it's here now so might as well push it forward to the community.