Add /api/users/:username/isAdmin route
Add /api/users/:username/isAdmin route

Returns a JSON object { isAdmin: (bool) } because a simple bool as a
JSON response may not be acceptable to all clients (it was not allowed
by RFC 4627, and though it's allowed by RFC 7159 it's best to avoid
returning simple bools for maximum interoperability.

sillsdev/web-languagedepot-api,sillsdev/web-languagedepot-api,sillsdev/web-languagedepot-api,sillsdev/web-languagedepot-api