Revise cookie compliance to meet EU regs

[delphidabbler]

Neither the new or old Osano cookie compliance code strictly met EU regulations.

Revise the code to better comply.

• The cookies used are preferences cookies. Strictly speaking the user can opt out of these. So a Yes/No choice is required. If no then the cookie compliance popup & any flash messages will always appear.

• Cookies can't be stored for > 1 year, so permanently hiding flash messages is not an option.

• Users decisions to opt out must be recorded - let's argue that this is done by storing the cookie.

• Information about each cookie used must be provided, so need to update cookies.html. For accuracy it may be a good idea to store each cookie's name in a Jekyll variable so same one gets used as actual name & in Docs.

• Users must be able to withdraw consent. Possibly add a link to click on page footer when user is opted in.

[delphidabbler]

Implemented at 85e2a51