Autocomplete compatibility

[alaric-dfe]

As per https://trello.com/c/wV2IRwEE/758-define-specifications-for-a-dropdown-compatible-reference-data-list

This PR formalises the concept of what structure a reference data list needs if it's to be put into an autocomplete-based selection thing such as dfe-autocomplete.

It:

• Provides documentation of what an autocomplete-compatible list is
• Marks what lists are autocomplete compatible (currently all of them!)
• Provides a predicate on a reference data list to confirm that it is autocomplete-compatible
• Adds RSpec tests that ensure all the supposedly autocomplete-compatible lists actually are
• Fixes a small synonym duplication in the institutions list, which was breaking its autocomplete compatibility (yay for automated testing)