Add new fields to the `links` index

[Chaphasilor]

• a field for the OD url (the scanned url, could also be a suburl because it contains the root url)
  this allows users to limit the search to specific ODs only, if they prefer that. it would also allow to provide a search link for a specific OD along with the scan results
• a field for the file extension.
  this makes it easier to filter certain types of files (we could create custom extension groups for video files, audio files, documents, etc., that we can select in advanced search, or have the user input the extension(s) manually)

[MCOfficer]

file extensions are done. i wonder what elasticsearch field type OD urls would be. keyword would be ideal because there are tons of duplicates in that field, but text is also an option.

[Chaphasilor]

As long as keyword can be used for exact matching, that should work...