Unified REST call to perform controlled term field auto-complete

[martinjoconnor]

Currently the front end must make a series of calls to the Terminology Server to assemble suggestions for controlled term fields.

Instead, develop a single REST call that simplified this lookup. Notionally, it will do the following:

[term1, term2, ...] <- f(<text typed by user>, field._valueConstraint)

Basically, when a user is populating a controlled term field in the Metadata Editor this call can be used to give auto-complete suggestions for the field based on the text typed by the user and the value constraint specification for the field.

Required by metadatacenter/cedar-metadata0-editor#963