[feature/frontend] Respect prefers-reduced-motion for avatars, headers, and emojis (#3118)

* [feature/frontend] Respect `prefers-reduced-motion` for avatars, headers, and emojis

* go fmt

* fix tests

* use static version of instance thumbnail when appropriate

* use prefers-reduced-motion

* simplify account conversion a bit

* fix c&p error

internal/api/model/attachment.go

@@ -107,6 +107,10 @@ type WebAttachment struct {
 	// MIME type of
 	// the attachment.
 	MIMEType string
+
+	// MIME type of
+	// the thumbnail.
+	PreviewMIMEType string
 }
 
 // MediaMeta models media metadata.