[#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.
[#76] Add extra fields to the source schema

Add 'private' and its core validators, and 'metadata_modified' and
'metadata_created'.

Also ignore '__extras'
[#76] Use harvest_source_show on reindex command
[#77] Use auth_allow_anonymous_access decorator

Starting from 2.2 you need to explicitly flag auth functions that
allow anonymous access with the p.toolkit.auth_allow_anonymous_access
decorator. A local version of the decorator is used to ensure we only
use it on CKAN>=2.2