Add script attribute to the cookie banner to mark additional cookies as essential.

[edthedev]

Context

Once we have #254, many users will reject all non-essential cookies.
This can lead to a degraded experience, for sites with additional functional cookies that are not known to the cookie banner.

It could be useful to add support for a data-essential-cookie attribute on the script tag that identifies an additional cookie to be saved when all other cookies are deleted.

Tasks

• add data-essential-coookie to one of our example pages
• update code from #254 to skip over a cookie identified as essential in this manner
• but ignore any attempt to save known non-essential cookies in this manner (such as analytics cookies from popular vendors). This feature must not be a work-around to treat tracking cookies as essential. Throw a console warning if a non-essential cookie is specified.
• add multiple copies of data-essential-coookie with different values to one of our example pages
• ensure that multiple cookies can be marked as essential in this manner