[fix] default_doi_resolver in preferences

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>