[#18] Update delete harvest source functionality

The harvest_source_delete logic function proxies to package delete,
which will delete the harvest source dataset. The harvest plugin then
hooks to the after_delete extension point in order to inactivate the
actual HarvestSource object and abort any pending jobs.
Also added the Delete button to the harvest source form.
[#18] Allow reactivation of sources

Due to #607 in CKAN core, once a source was deleted you could not
reactivate it again. As a workaround, if the source is deleted the
Delete button is not shown and the state select is, so you can set it to
'active'.
Also fixed wrong redirect after deletion.
[#8] Update how state is handled for source objects
Merge branch '18-delete-sources-for-2.0' into release-v2.0