GitLab

Add the ability to undo add, edit, and delete for custom links. Also
added separate notifications for each action.

Screenshots:
https://screenshot.googleplex.com/rmGyjvf4AD9.png
https://screenshot.googleplex.com/fvp9f4zFMJ8.png
https://screenshot.googleplex.com/7FRRUa3OGuN.png

Bug: 856394, 851293
Change-Id: I42f218e6ad2f4b384491ddc6abf26cabd23afb18
Reviewed-on: https://chromium-review.googlesource.com/1155633Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579949}