Add support for sub-header text in Login component

[mcoker]

Related to #354

Need to add support for the "Log in to Application" text as seen on https://rawgit.com/patternfly/patternfly/master-dist/dist/tests/login-single-sign-on.html

Details to come.

cc @tlabaj @mceledonia

[mceledonia]

Let's go with small text style (14px) left-aligned underneath the header with spacing: 16px top, 32px below.. on mobile the language selector should go below the description and above the inputs.

[mcoker]

@mceledonia the bottom spacing is a bit different. After the header text currently, there is 16px bottom padding on the header, then the hidden error message, and then 24px (lg) spacer as the gap between the error message and first form field.

Here I just left all of the existing spacing the way it is and added the sub-header text (description) with 16px between it and the content above it. Then on mobile, there is 16px between the title, description, and dropdown.

Desktop

Desktop with error

Mobile

Mobile with error

I think it looks OK, but the spacing between the description and error on desktop seems too tight. Here it is with an 8px (sm) spacer below the description on desktop only.

Regular

Error

[mceledonia]

This all looks great, thanks @mcoker !!