[feature] Implement the preferences client API (#1740)

This adds the preferences endpoint to our Mastodon Client API
implementation. It's a read-only endpoint that returns a number of
user preferences. Applications can query these settings when logging in
a user (for the first time) to configure themselves.

testrig/testmodels.go

@@ -487,8 +487,8 @@ func NewTestAccounts() map[string]*gtsmodel.Account {
 			Locked:                  TrueBool(),
 			Discoverable:            FalseBool(),
 			Privacy:                 gtsmodel.VisibilityFollowersOnly,
-			Sensitive:               FalseBool(),
-			Language:                "en",
+			Sensitive:               TrueBool(),
+			Language:                "fr",
 			URI:                     "http://localhost:8080/users/1happyturtle",
 			URL:                     "http://localhost:8080/@1happyturtle",
 			FetchedAt:               time.Time{},