Use class instead of enum for `ConferenceTimezone`
 - enums are too tricky since the web client wants the America/New_York value instead of EASTERN
 - using an enum would mean lots of extra code for String -> enum -> String both at the DB and Jackson layer
 - this solution is more streamlined
Use class instead of enum for `ConferenceTimezone`
 - enums are too tricky since the web client wants the America/New_York value instead of EASTERN
 - using an enum would mean lots of extra code for String -> enum -> String both at the DB and Jackson layer
 - this solution is more streamlined
move validation logic
 - into service layer, out of process layer
 - null/empty string is okay
 - update migration script to have tz code
Merge branch 'EVENT-45' into dev

Conflicts:
	src/main/java/org/cru/crs/api/model/conference/ConferenceTimezone.java
	src/main/java/org/cru/crs/api/process/CreateConferenceProcess.java