feat: add warning form message type and invalid input class

[charlieTheBotDev]

Description

Adds:

• hx-form-message-warning - A warning message for forms, used generally to warn before the user enters data about format requirements etc.
• hx-input-invalid (for all hx-input-xx elements) - A class to manually set the visual state of an input to invalid, for example if the value input fails validation.
• Remove some leftover code from form/index.css that was causing the deprecated form styles to show error messages with red text on a black background instead of white.

Motivation and Context

UX improvements around forms. A release for the FormBuilder will add support for adding warnings to fields as well as supporting 'required' fields that show as invalid only after a user has entered data

How Has This Been Tested?

CSS Only - Visual tests, all existing tests pass

Screenshots (if appropriate):

Checklist:

• [x] I have added a tag to this pull request that indicates the impact of the change (patch, minor or major)
• [x] My code follows the code style of this project.
• [x] I have updated the documentation accordingly. (This includes updating the changelog).
• [x] I have read the CONTRIBUTING document. (Last updated 16 Aug 2019)
• [x] All my changes are covered by tests.
• [x] This request is ready to review and merge

[coveralls]

Coverage decreased (-0.04%) to 69.673% when pulling 758ca3a386301658de3276f62b25f3532bb3e104 on feature/form-validation into 17b84d2d87ceac2c8f94893da237c28c2e1ba735 on develop.