Don't allow multiple Campaigns having the same CampaignPlan

Created by: eseliger

Created by: mrnugget

Fixed the tests and an actual bug in the CreateCampaign method that caused the transaction to not be rolled back properly.

Created by: codecov[bot]

Codecov Report

Merging #8991 into master will decrease coverage by <.01%. The diff coverage is 58.82%.

@@            Coverage Diff             @@
##           master    #8991      +/-   ##
==========================================
- Coverage   41.72%   41.71%   -0.01%     
==========================================
  Files        1314     1314              
  Lines       70841    70847       +6     
  Branches     6556     6556              
==========================================
+ Hits        29556    29557       +1     
- Misses      38592    38595       +3     
- Partials     2693     2695       +2

Impacted Files	Coverage Δ
enterprise/internal/campaigns/service.go	36.1% <58.82%> (+0.68%)
cmd/repo-updater/repos/github.go	69.31% <0%> (-0.66%)