Version 1.0.0

• Add support for Rails 6.1 - thanks to @gpeng #79

A brief summary of everything that's included thus far:

• Components:
  • Back link
  • Breadcrumbs
  • Details
  • Footer
  • Header
  • Inset text
  • Notification banner
  • Panel
  • Phase banner
  • Start now button
  • Summary list
  • Tabs
  • Tag
  • Warning text
• Helpers:
  • #govuk_link_to
  • #govuk_mail_to
  • #govuk_button_to
  • #govuk_back_to_top_link
  • #govuk_skip_link
• Access to components via friendly helpers, e.g., #govuk_panel