Update textarea config

[Rhodine-orleans-lindsay]

What

• Format textarea to follow govuk design more closely and extend it to include word count
• Add word count validator
• Amend ReadMe to include 'maxword' option
• Amend format of question page heading in sandbox for consistency

  Why

• To follow govuk design guidelines
• To account for services that may use word count instead of character count

  How

• Amended template-mixins.js to configure 'maxword' option
• Amended textarea-group.html
• Added 'maxword' validator to validator.js

  Testing

  Tested locally