Alert > Add sticky property

Proposal

Add a sticky property that can be applied to the forthcoming alert error variant.

This was initially added in !1910 (merged), but that MR had to be reverted because it didn’t take into account the HAML and Ruby templates.

Scope

The sticky property could apply to any alert variant, however the recommendation in Pajamas is that only the critical alerts would have the property set to true. This would also be reenforced in GitLab UI through examples any by including the changes from !1910 (diffs).