Don't auto-tag partial matches.

Add `import: confirm_partial` setting, with a default of "no" (current
behaviour). If set to "yes", strong recommendations for partial matches will
be downgraded to medium recommendation, thus will require confirmation and
will not be auto-tagged.
Safer defaults on confirmation prompts.

Remove default option and require selection on confirmation prompts for:

- Partial matches, if `import: confirm_partial` setting is "yes".
- Matches that are below the medium recommendation threshold, but above the
  gap threshold.
- Matches that have no recommendation.
- Matches other than the best and auto-suggested match.
For matches with no recommendation, import as-is or skip automatically.

Add `import: none_rec_action` setting with a default of "ask" (current
behaviour). If set to "asis" or "skip", matches with no recommendation will be
imported as-is or skipped automatically.