Create a workflow validator to require a field on workflow transition

A popular feature request is that JIRA should be able to require a field only on a specific transition (JRA-5783). Currently field required'ness is determined by field configurations which are scoped by project/issue type.

An often suggested workaround is to create a validator which checks if a field is empty, and complains if so. There is even user-contributed code that does the job at:

http://confluence.atlassian.com/display/JIRAEXT/Workflow+Validator+to+require+a+field+on+a+transition+screen

Since it's so useful, we should package this properly as a plugin and distribute it with JIRA, as an 80% interim solution.