Replace utilitybeltassertions and assertthat with selkamand/assertions

[selkamand]

2 deps down to 1 and better error messages.

Also make sure to replace utilitybeltshiny::assert_reactive with assertions::assert_reactive to help eliminate the utilitybeltshiny dependency

[selkamand]

All calls to utilitybeltassertions now have drop-in replacements in the assertions package - we're well positioned to do this to drop the dependency