Fix missing scheme on default exporter

[kfdm]

• Move some fields to a BaseExporter class that can be used by both
  DefaultExporter and Exporter to keep the fields synced

[codecov-io]

Codecov Report

Merging #259 into master will increase coverage by 0.15%.
The diff coverage is 30.76%.

@@            Coverage Diff             @@
##           master     #259      +/-   ##
==========================================
+ Coverage   47.11%   47.26%   +0.15%     
==========================================
  Files          54       55       +1     
  Lines        2630     2630              
==========================================
+ Hits         1239     1243       +4     
+ Misses       1391     1387       -4

Impacted Files	Coverage Δ
promgen/models.py	29.03% <0%> (+0.3%)	⬆️
promgen/migrations/0018_default_exporter_scheme.py	100% <100%> (ø)

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