Describe the bug
At the moment, submissions can be created via 3 mechanisms:

• Normal workflow
• QuickSubmit plugin
• Import/export toolset

Later, it's sometimes necessary to try to distinguish between these sources using heuristics. For example:

• In the stats toolset, QuickSubmit-based submissions (and possibly import/export ones too) are excluded from editorial stats to avoid a disproportionate effect on e.g. days to publication;
• In the Google Scholar plugin, there may be a related heuristic around issue and submission publication dates (though I'm not 100%
  sure it's quickSubmit/importExport causing the issue).

Recommendation:

• Add a column to the submissions table, called e.g. "source", allowing a plain-text indication of the submission's origin.
• For new submissions populate this based on the use case.
• On upgrade, use the existing (e.g. stats) heuristic to determine the source. This might not be 100% correct but that's no worse than status quo.
• Remove the stats heuristic and have it depend on the source column instead.

Suggested vocabulary for the column:

• nativeImportExport for submissions that were imported via the native import/export plugin
• quickSubmit for submissions that were created by the quickSubmit plugin
• workflow for submissions that were created using the normal editorial workflow
• quickSubmit-heuristic for submissions that were upgraded from a version without the source column, but appear by heuristic to have been created using quickSubmit
• unknown-upgrade for submissions that were upgraded from a version without the source column, and didn't appear to be created using quickSubmit by the heuristic