DT-458: Add ability for admins to export stories regardless of status

[torresga]

Jira Ticket

https://ombulabs.atlassian.net/browse/DT-458

Motivation / Context

Currently, stories are only exported to CSV if they have an accepted status. This feature allows admin users to export stories regardless of their status. Non-admins should only be able to export approved stories.

I also refactored the export method because Code Climate was complaining.

QA / Testing Instructions

1. Ensure the user you are using to test is an admin.
2. Go to the project that has stories you'd like to export
3. Click on 'CSV Import/Export'
4. Click on the 'Export all stories' checkbox
5. Ensure the resulting CSV has all stories regardless of status

Screenshots:

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I will abide by the code of conduct.