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.