WIP: Improve backup system

[Spitfireap]

Since rspamd is not in PRIMARY_APPS, the backup system doesn't use it.
This adds antispam_apps in the backup process.

[codecov]

Codecov Report

❌ Patch coverage is 44.44444% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.91%. Comparing base (73feb96) to head (24ca07f).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
modoboa_installer/utils.py	42.85%	12 Missing ⚠️
run.py	55.00%	9 Missing ⚠️
modoboa_installer/scripts/__init__.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #607      +/-   ##
==========================================
- Coverage   54.03%   53.91%   -0.13%     
==========================================
  Files          12       12              
  Lines         879      881       +2     
==========================================
  Hits          475      475              
- Misses        404      406       +2     

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[almereyda]

Can this be rebased and checked?

The wonderful rspamd work has since been merged.

[Spitfireap]

Will do