Verify keycloak token

[grzanka]

Front end should be partially responsible to check if the user has access to the yaptide platform.
Add appropriate check which extracts preferred_username from the token.
If it is equal to plgkongruencj (hardcode it!) then display a message that the user doesn't has access to the yaptide.
Then lead the user to the yaptide main page (forced logout?)

If the username is different than plgkongruencj then login the user as usual.

The check based on username will be replaced by some additional fields in token, to be provided soon.