Improve .gitlab-ci.yml

This MR improves .gitlab-ci.yml by:

• Break out CI test:install and test:update logic into scripts under ./support/ci which reduces duplication, adds some informative echo calls and is easier to follow (IMHO) 🙂
• Performs a more accurate gdk update in the test:update job by first checking out master & installing and then checking out the CI SHA & updating
• Use gdk install shallow_clone=true which improves git clone speed and reduces CI time.
• When calling curl against the running GDK to verify if everything worked OK, use http://127.0.0.1:3000/users/sign_in as it's a more accurate test URL (http://127.0.0.1:3000/users/sign_in is a HTTP 200 vs. http://127.0.0.1:3000 being a HTTP 302).
• Adding explicit stage: test to some jobs that were missing.

Closes #555 (closed)