[1.x] Improve React components

[jessarcher]

This PR makes the following changes to the React components (and their usage throughout the application):

• Additional props are now passed to the underlying HTML element (similar to Vue). This allows us to remove explicit props for things like id, autoComplete, and required.
• The forInput prop has been removed in favour of passing the standard JSX htmlFor through.
• The handleChange prop on TextInput has been removed in favour of passing the standard JSX onChange through.
• The processing prop on buttons has been changed to disabled to reflect better what it's doing and also to match the Vue stack.
• All components customising the className of an underlying element now accept a className prop that will be appended to the underlying element. Previously only some components were doing this.
• Explicit props that specify a redundant default (i.e. the same default as the underlying component) have been removed.