Upgrade jQuery to v2.x (#2246)

Fix #2199 and minifying assets file.

jQuery v1.x is supports legacy browsers.
But, Mastodon does not support legacy browsers.
I think jQuery v1.x is unnecessary.

app/assets/javascripts/application.js

@@ -10,6 +10,6 @@
 // Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
 // about supported directives.
 //
-//= require jquery
+//= require jquery2
 //= require jquery_ujs
 //= require components

app/assets/javascripts/application_public.js

-//= require jquery
+//= require jquery2
 //= require jquery_ujs
 //= require extras
 //= require best_in_place