Adds style for warnings and notes

[joaofnfernandes]

This PR adds a way for us to have warnings and notes.

It also maintains backwards compatibility with the older note style so that we can start migrating slowly (the first note in the screenshot).

[johndmulhausen]

This is great! Can you do a yellow/gold variant for "Caution"/"Be Careful" messages?

[joaofnfernandes]

I'd prefer to have just two colors, for the reasons pointed in #2563, but we can create 3 colors and. @jsouth what hex colors should we use for these banners? We need colors for:

• info
• caution
• warning

Right now I'm using the colors of the bootstrap docs but I'd prefer if we had our own colors.

[joaofnfernandes]

Now we have 3 colors. You can use regular blockquotes for the default blue, or add the class warning for yellow/brown, or danger for red.

[jsouth]

@joaofnfernandes - I'll collect these intent colors today and post here :)

[joaofnfernandes]

Merging. We can iterate the colors of a new PR.