feat: Keycloak support added

[ip6li]

This PR adds support for a local Keycloak server, which also adresses #291.

Please note that some files from old GUI needed to be touched:

• src/main/resources/i18n/messages.properties
• src/main/resources/i18n/messages_de.properties
• src/main/resources/static/css/alovoa.css
• src/main/resources/templates/login.html
• src/main/resources/templates/register.html

This may interfere with new GUI.

[Nonononoki]

What are the advantages of Keycloak when you only have one applications?
What are the reasons why other dating platforms don't use Keycloak?

[ip6li]

Keycloak is an Open Source identity provider solution. It support OpenID/Connect, OAuth2 and SAML. OAuth2 is used for this Alovoa PR. Keycloak allows using many backends even Active Directory is supported.

I do not know if other dating platforms are using Keycloak as OpenID IDP, but some of them using Okta, AWS IAM OIDC or Azure IAM.

In my case Alovoa is not the only one application controlled by Keycloak IDP, same Realm is used for a Matrix server. This Matrix server will be used for an alternative chat solution at Alovoa (Element Web). Even if Keycloak is used for Alovoa only it has many advantages, e.g multi factor authentication works out of the box.