Fix exporter test

[kfdm]

• Query based on project.id (instead of farm.id) for future refactoring
• Change from form to regular POST data to avoid extra validation confusion
• Simplify exporter-test button html

[codecov-io]

Codecov Report

Merging #263 into master will decrease coverage by 0.05%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master     #263      +/-   ##
==========================================
- Coverage   47.20%   47.14%   -0.06%     
==========================================
  Files          56       56              
  Lines        2646     2647       +1     
==========================================
- Hits         1249     1248       -1     
- Misses       1397     1399       +2     

Impacted Files	Coverage Δ
promgen/urls.py	94.44% <ø> (ø)
promgen/views.py	47.78% <72.72%> (-0.22%)	⬇️

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