Add a clear button for file attachment and validate the form client-side

[culshaw]

This issue is designed to give us some insight into how you work, and how well you might fit into our team, and our workflow. This is more about how you code.

Please see the main README in this project for a full test spec.

Requirements

Clear button for file attachment

When a file is added to the form we show the filename but there's currently no way to remove the attachment you have selected. Add support for this, with tests.

Validate the form client-side

Currently the form can add messages to the conversation stream without validation - most notably empty messages, but there are other aspects that can be tightened up too. Add support for this, with tests.

CSS

• Demonstrate the ability to add a javascript trigger following the CSS animation (an alert or console.log() is enough)
• Add commentary to our Stylus (in styls/) to advise on best practices

Submit your application as a pull request.