fix(GlTextArea): Set SSOT for styles

A tiny MVC for #755 (closed) where we want to make sure that the GlTextArea matches the design guidelines: https://www.figma.com/file/qEddyqCrI7kPSBjGmwkZzQ/Pajamas-UI-Kit?node-id=425%3A10

This is done by ensuring we set gl-form-input as our default textbox style class and only use gl-form-textarea for controlling textarea specific styles such as height. This creates a SSOT for the textbox styles in:

src/components/base/form/form_input/form_input.scss

Screenshots