GitLab

- migrate many methods into the handler
- update loading/error screen specs

Bug: b/78190629
Test: Manual
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I7a590f547cc85309f982c58421f0984721a4f04b
Reviewed-on: https://chromium-review.googlesource.com/1112716Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570113}