Added access to scan helper function to better scale exports #32

[TeriForey]

Description of Change

Large exports are now dealt with the elasticsearch helper scan function instead of search.

Issues Solved

#32

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[azerbini]

@TeriForey finally got round to testing this for the first time this morning. I am unfortunately still experiencing issues with this. I tested this on our North African db (see URL in the wetransfer SQL dump I just sent you) on the search below and exporting to CSV, but I get the usual '504 Gateway Timeout The gateway did not receive a timely response from the upstream server or application'. Could you perhaps test this code again against the SQL dump I just sent you, bearing in mind to checkout the arches-eamena-stable branch for your testing. Cheers!