[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.

internal/api/model/preferences.go

@@ -36,4 +36,6 @@ type Preferences struct {
 	ReadingExpandMedia string `json:"reading:expand:media"`
 	// Whether CWs should be expanded by default.
 	ReadingExpandSpoilers bool `json:"reading:expand:spoilers"`
+	// Whether gifs should automatically play.
+	ReadingAutoPlayGifs bool `json:"reading:autoplay:gifs"`
 }