Improved devcontainer environment

[ajparsons]

This PR adds the basic boilerplate for a devcontainer setup to work in VSCode or Codespaces.

This is to make it easier to test changes to help text, donation messaging, surveys, etc.

This includes:

• Github Action to mirror back to git.mysociety.org
• Loading Ruby/Compass as part of container creation (doesn't change existing docker instructions)
• Modification to code to fix the issue referenced in the readme that 'localhost' can't be used as a URL in development environment.
• Extra catch around import of PHPMailer to remove an error that prevents reviewing the content of help pages, etc.

In principle, this shouldn't have any impact on other dev environments or how it works in production - but those last two points need a check.