Branch list enhancements (#808)

[6543]

Backport #808

• Allow users not logged in to access branches page
  (fixes a nil pointer derefernce)

• Add Gogs support for branches

[codecov-commenter]

Codecov Report

Merging #809 (cb53342) into release/v0.15 (ce07632) will decrease coverage by 0.20%.
The diff coverage is 0.00%.

@@                Coverage Diff                @@
##           release/v0.15     #809      +/-   ##
=================================================
- Coverage          51.71%   51.51%   -0.21%     
=================================================
  Files                 88       88              
  Lines               5996     6020      +24     
=================================================
  Hits                3101     3101              
- Misses              2717     2741      +24     
  Partials             178      178              

Impacted Files	Coverage Δ
server/remote/gitea/gitea.go	37.85% <0.00%> (-0.49%)	⬇️
server/remote/github/github.go	16.11% <0.00%> (-0.19%)	⬇️
server/remote/gitlab/gitlab.go	27.08% <0.00%> (-0.28%)	⬇️
server/remote/gogs/gogs.go	56.98% <0.00%> (-3.94%)	⬇️

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