[server] Add workaround for intermittent Safari CORS errors (#4540)

At times, Safari will fail our API responses because its CORS preflight
fails. The errors are reproducible, but intermittently, and not on
localhost. We seem not to be the first ones to hit
[this](processing/p5.js-web-editor#3156).

Based on a hint from:


supabase/supabase#20982 (comment)

Modify our CORS responses to use 200 instead of 204 to try and fix
Safari.

server/cmd/museum/main.go

@@ -1007,7 +1007,9 @@ func cors() gin.HandlerFunc {
 		c.Writer.Header().Set("Access-Control-Max-Age", "1728000")
 
 		if c.Request.Method == http.MethodOptions {
-			c.AbortWithStatus(http.StatusNoContent)
+			// While 204 No Content is more appropriate, Safari intermittently
+			// (intermittently!) fails CORS if we return 204 instead of 200 OK.
+			c.Status(http.StatusOK)
 			return
 		}
 		c.Next()