Changes:

- `via` is not required in FastAPI. We deduce the involved
  requests via their PackageBaseName / MergeBaseName columns
  and set them to Accepted when merged.
- When erroneous input is given, the error is now presented
  on the merge page instead of sending the user to the pkgbase
  page.

Signed-off-by: Kevin Morris <kevr@0cost.org>