Pajamas-compliant alert HAML template

What does this MR do?

This updates the shared alert HAML template as follows:

• Titles are now wrapped in H4's instead of DIVs.
• Dismiss button now...
  • ...follows the design specs by using tertiary GlButton styles.
  • ...can be hidden by setting the dismissible parameter to false (true by default).
• Alerts content width now uses GitLab UI's width constraint when using a fluid layout.
• With a fixed layout, alerts use GitLab's content classes so that the alert's content width matches that of the primary content. This can be opted-out by setting the is_contained option to true, which should be done when rendering an alert inside the primary content.

Additionally:

• Some info buttons have been migrated to the confirm variant (no visual change).
• The ee/app/views/shared/_new_user_signups_cap_reached_alert.html.haml view has been migrated to use the shared alert template (re #325409 (closed)).

Screenshots (strongly suggested)

Fluid layout

Before	After

Fixed layout

Before	After

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.