generalise the registration link in navbar solution from @jdblischack to work across all ticket providers

[DaveParr]

in #45 @jdblischak proposed a solution to generalise based on modifying the section ID.

Another idea: change the template files for the registration providers so that they all use id="registration". Then the navbar link could be shared between them, i.e. href="{{ .Site.BaseURL }}#registration">. I think in the edge case of the template where multiple registration vendors are enabled, the link would take you to the first div with a matching id.

I've implemented this along with an OR statement in the nav bar to tirgger the link being available or not.

[DaveParr]

Just realised I've made an error in the nav.html. Reworking now.

[jdblischak]

I didn't explicitly test it out, but this does implement my suggestion, so I approve! Thanks @DaveParr!