Support adding exporter defaults from admin gui

[kfdm]

• Migrate from using a settings file to register default exporters to managing it via the django admin to ease management tasks.
• Provide a register-exporter CLI subcommand for use from configuration management
• Rename register to register-server to distinguish commands

[codecov-io]

Codecov Report

Merging #114 into master will decrease coverage by 0.05%.
The diff coverage is 31.25%.

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
- Coverage      45%   44.94%   -0.06%     
==========================================
  Files          36       37       +1     
  Lines        2324     2336      +12     
==========================================
+ Hits         1046     1050       +4     
- Misses       1278     1286       +8

Impacted Files	Coverage Δ
promgen/views.py	43.85% <ø> (+0.08%)	⬆️
promgen/models.py	25.29% <0%> (-0.53%)	⬇️
promgen/admin.py	0% <0%> (ø)	⬆️
promgen/migrations/0006_exporter_options.py	100% <100%> (ø)
promgen/context_processors.py	100% <100%> (ø)	⬆️

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[m-tkg]

I found Makefile contains some tabs after .PHONY.