Make ckanext-wet-boew more future-proof with CKAN

Hullo there,

This is a pull request that makes ckanext-wet-boew a little more future-proof with CKAN core as it takes advantages of new template blocks (coming to CKAN master soon), `{% ckan_extends %}` and `{{ super() }}` wherever it can. Using these techniques I've reduced ~5000 lines of template code down to ~1000.

The new template blocks on CKAN core haven't been merged yet [1] so in order to test this properly with CKAN please `git checkout 1301-template-blocks` on your dev CKAN instance. Eventually the changes to core will end up in master and most probably CKAN 2.3.

I think I've managed to get every template WETified, but do give me feedback and notes on any page/template that I've missed and I'll update the pull request accordingly.

[1] https://github.com/okfn/ckan/pull/1301