Existing accessibility issues from main repo

[ajb]

accessibility - colors contrast ratio twbs#3572

add icons to alert messages twbs#1453

accessible tooltips: twbs#8469

[bensheldon]

Just to capture that there are some alignment issues between Bootstrap's maintainers' priorities/intent and accessibility.

on accessible tooltips: "dont want to switch the css to css attr's..."

on colors contrast ratio: "I want to ensure that readability and style are maintained."