Add custom disconnect message function

[chfoster]

Adding the customDisconnectMessage to the dfeshiny package, which shows the following overlay when RSConnect would previously have 'grey-screened'.

Documentation still needs updating: So refresh needs arguments entry and need to add examples etc

[chfoster]

I've checked that using this branch in the shiny-template works as expected (there must be an issue with the test dashboard in this repo that means it displays differently to the template):

[cjrace]

dfeshiny custom disconnect message