Customisable disconnect message

[JT-39]

Is your feature request related to a problem? Please describe.

The dfeshiny::custom_disconnect_message() function is quite restrictive in its content and geared towards EES. Below is a screenshot.

Describe the solution you'd like

It would be useful to have the following:

• Allow the alternative site to be the the sites plain link (e.g., https://department-for-education.shinyapps.io/local-authority-interactive-tool/)
• Be able to link a data source site that is GitHub
• Any other text or an email different from [email protected]?

Describe alternatives you've considered

Additional context