[Issue #3579] Update opportunity CSV generation

[chouinar]

Summary

Fixes #3579

Time to review: 5 mins

Changes proposed

Remove a few columns from the opportunity CSV extract that shouldn't be used (bad data / duplicative)

For the search endpoint, add a maximum to the page size to prevent someone from using it to fetch everything (if they want that, we have extract files which will be better).

Slight adjustment to CSV writing logic

Cleaned up some old unused pagination code from an earlier implementation

Context for reviewers

For fields removed:

• Agency == agency code, but been moving us to only use agency code for clarity on what the field actually is, so removing the extra duplicate
• Agency phone number has bad data and we already removed it from the search API itself

The small adjustment to the CSV writing logic is to have one less iteration over opportunities in the endpoint. No idea if it'll add much performance gain, but will use less memory/cycles for an easy change.

[mdragon]

Looks good