[Brent] Add Staff Role column to csv export

[MorayMySoc]

If a report is by a member of staff then check if they have a role and set that in the exported data.

https://mysocietysupport.freshdesk.com/a/tickets/4306

[skip changelog]

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.71%. Comparing base (b595c61) to head (2a53c76).
Report is 2 commits behind head on master.

Files	Patch %	Lines
perllib/FixMyStreet/Cobrand/Brent.pm	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5042      +/-   ##
==========================================
+ Coverage   82.67%   82.71%   +0.03%     
==========================================
  Files         396      396              
  Lines       30821    30948     +127     
  Branches     4889     4939      +50     
==========================================
+ Hits        25481    25598     +117     
- Misses       3895     3900       +5     
- Partials     1445     1450       +5     

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