Styling refresh of disconnect dialogue

[rmbielby]

Brief overview of changes

I've adapted the disconnect dialogue box to make it a little more user friendly;

• more GDS consistent styling
• clearer, less jargony text
• aria labelling

Why are these changes being made?

CSC team suggested some changes to the disconnect message dialogue box to make it less imposing for users:

• Nicer colour scheme
• Less technical text (e.g. mirror etc)

Detailed description of changes

• Replaced message text with something a little clearer.
• Updated css to be more GDS consistent - white box with a focus-yellow border
• I've shifted the css to be attached within the package itself rather than relying on the contents of dfe_shiny_gov_style.css
• Pointing to an EES publication is now optional (the CSC dashboard and some others collate data from across several EES and non-EES data sources, so offering to send the user to EES or a single publication EES doesn't work in this context)

Additional information for reviewers

We may need / want to clear out the old css from the copied dfe_shiny_gov_style.css files some dashboards.

Issue ticket number/s and link

...

[cjrace]

Couple of linting issues from lintr and then extra comments from me.

Also probably worth adding some testing on the HTML output, so checking things like the aria role is set right, the links pull through and are in the position expected etc?

This also should resolve #32 - so should either link that or mention it's fixed / resolved in the description

[cjrace]

Happy to approve, want to do it as a patch version update?