[#70] Fix Add harvest source button not showing

Due to changes in the templates starting on 2.1 the add source button
was not showing. The whole search template has been simplified,
separating in a separate file the 2.0 only code.

Tested in 2.0, 2.1 and 2.2
Merge branch 'master' of github.com:okfn/ckanext-harvest
Fix resource updating for harvested datasets

Starting from 2.2, resource_update calls package_show before updating
the resource via a package_update call. The dict passed had the harvest
extras (eg harvest_object_id) added which made the update call fails due
to duplicated extra keys. To fix it we now remove any harvest extras
on after_show if there is a 'for_edit' property on the context.