[7942] Add empty CSV template for bulk trainee upload

[stevehook]

Context

Add the link for the empty bulk trainee upload CSV template.

Changes proposed in this pull request

• Store the sample file given in the requirements
• Serve this file from a new controller with attachment disposition
• Extend feature spec to cover the new route

Guidance to review

Important business

• Does this PR introduce any PII fields that need to be overwritten or deleted in db/scripts/sanitise.sql?
• Does this PR change the database schema? If so, have you updated the config/analytics.yml file and considered whether you need to send 'import_entity' events?
• Do we need to send any updates to DQT as part of the work in this PR?
• Does this PR need an ADR?

NB: Please notify the #twd_data_insights team and ask for a review if new fields are being added to analytics.yml

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 96.09%. Comparing base (943e89f) to head (fa440b9).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
..._update/add_trainees/empty_templates_controller.rb	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4906      +/-   ##
==========================================
- Coverage   96.10%   96.09%   -0.02%     
==========================================
  Files         743      746       +3     
  Lines       16050    16078      +28     
==========================================
+ Hits        15425    15450      +25     
- Misses        625      628       +3     

Flag	Coverage Δ
unittests	96.09% <66.66%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud