REST POST service /classifier/classes/update to update an existing data class

[turnerrainer]

AS AN Architect
I WANT TO have a REST service to update existing data classes
SO THAT this would be the only generic functionality of its kind

Acceptance Criteria

• REST POST endpoint /classifier/classes/update
• Add new sample datasets for an existing data class
• Update the regex rules (e.g. email, UTF-8, etc.) that the data class is linked to
• REST GET endpoint /classifier/classes/state to return the current state of a defined data class #115
• Confirm mock request schema

Additional requirements

• Any updates on data classes are only allowed when the given data class isn't in use in any active dataset groups #116