login screen texts

https://issues.jboss.org/browse/AGPUSH-701

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This resolves AGPUSH-701, but in this PR, we had to introduce own copy of `login.ftl` template from Keycloak which establishes unhealthy copy&paste updating approach.

I have opened https://issues.jboss.org/browse/KEYCLOAK-548 to resolve that.

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Additionally I don't like bad readability of upper-cased `LOG IN TO UNIFIEDPUSH SERVER` because of joined words in `UNIFIEDPUSH`