DEV: Move Discourse.getURL and related functions to a module (#9966)

* DEV: Move `Discourse.getURL` and related functions to a module

* DEV: Remove `Discourse.getURL` and `Discourse.getURLWithCDN`

* FIX: `get-url` is required for server side code

* DEV: Deprecate `BaseUri` too.

test/javascripts/lib/computed-test.js

@@ -9,6 +9,7 @@ import {
   url,
   htmlSafe
 } from "discourse/lib/computed";
+import { setPrefix } from "discourse-common/lib/get-url";
 
 QUnit.module("lib:computed", {
   beforeEach() {
@@ -148,7 +149,7 @@ QUnit.test("url", assert => {
     "it supports urls without a prefix"
   );
 
-  Discourse.BaseUri = "/prefixed";
+  setPrefix("/prefixed");
   t = testClass.create({ username: "eviltrout" });
   assert.equal(
     t.get("userUrl"),