Instead of a hard-coded `oadoi.org` default, use the default value from
`settings.yml`.
Fix an issue in the themes: The replacement 'current_doi_resolver' contains the
doi_resolver_url, not the name of the DOI resolver. Compare return value of::
searx.plugins.oa_doi_rewrite.get_doi_resolver(...)
Fix a typo in `get_doi_resolver(..)`: suggested by @kvch:
*L32 should set doi_resolver not doi_resolvers*
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
ebfd0eb2b7