Bug 35346: Update template logic to display 'Essential'

Bug 35346: Update template logic to display 'Essential'

This patch updates the cookie consent template logic to always display
the 'Accept essential cookies' button and only expose the 'Accept all
cookies' button when there are additional cookies to accept, i.e. when
JSConsents is populated.

Test plan
1) Enable CookieConsent
2) Note you see 'Accept essential cookies' in both OPAC and Staff client
   until you have accepted them
3) Add a cookie to ConsentJS
4) Note that you will now see 'Accept all cookies' as well as the 'Accept
   essential cookies' button.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>