Miscellaneous continuous export cleanup

[arsalansufi]

Will open standalone issues for relevant items when I wrap up my current stretch work on continuous export.

• Remove this mocking Remove the mocking of safeParseJson in artifact_authentication.test.ts #4100
• Have BallotImage field consistently include both a front and back image, when populated
• Revisit this TODO
• Use *File vs. *FileContents more consistently in variable names
• Fix flaky tests
• Only flush to USB once instead of after every write when performing a full export Only flush to USB once instead of after every write when performing a CVR export operation #4093
• Remove cvrs_backed_up_at tracking, after verifying that it's no longer used (check both VxScan and VxCentralScan)
• Make waitForContinuousExportToUsbDrive test helper in VxScan less arbitrary
• Recovery from mid-CVR-write failures
• Don't allow polls close if a mismatch is detected
• Don't allow clicks outside progress modals to dismiss them
• Add an indication of whether CVR exports are pre- or post-polls-close Mark pre-polls-close VxScan CVR exports in the VxAdmin CVR import UX #4094
• Determine if batchLabel field is still needed and remove if possible Determine whether the batchLabel field on the AcceptedSheet type is still needed and remove if possible #4095
• Have recovery logic export only what needs to be exported instead of everything Improve continuous export failure recovery logic #4096
• Update code names and variables to no longer refer to CVRs as single files Update variable names, etc., to no longer refer to CVRs as single files #4097
• Update CVR fixture generator to use actual CVR export functions Update CVR fixture generator to use actual top-level CVR export functions instead of recreating the logic #4098
• Re-enable CVR auth in CVR tests No longer skip CVR authentication in tests where possible #4101
• Export uninterpretable rejected ballots Support exporting uninterpretable rejected ballots #4099

[arsalansufi]

Closing this issue out now that I've created standalone issues for the outstanding checklist items