Add method to check organization membership

[lafriks]

Needed for right checks for organization wide methods

[codecov-commenter]

Codecov Report

Merging #1037 (fc62a6d) into master (1e8d4cc) will decrease coverage by 0.83%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1037      +/-   ##
==========================================
- Coverage   51.27%   50.44%   -0.84%     
==========================================
  Files          81       81              
  Lines        6157     6249      +92     
==========================================
- Hits         3157     3152       -5     
- Misses       2817     2914      +97     
  Partials      183      183              

Impacted Files	Coverage Δ
server/model/perm.go	0.00% <ø> (ø)
server/remote/bitbucket/bitbucket.go	86.87% <0.00%> (-2.43%)	⬇️
server/remote/bitbucketserver/bitbucketserver.go	0.00% <0.00%> (ø)
server/remote/coding/coding.go	63.84% <0.00%> (-0.92%)	⬇️
server/remote/gitea/gitea.go	35.38% <0.00%> (-1.86%)	⬇️
server/remote/github/github.go	15.71% <0.00%> (-0.31%)	⬇️
server/remote/gitlab/gitlab.go	24.31% <0.00%> (-2.64%)	⬇️
server/remote/gogs/gogs.go	52.73% <0.00%> (-3.65%)	⬇️
server/logging/log.go	48.83% <0.00%> (-5.82%)	⬇️

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[anbraten]

For caching I have created a separate issue as this could be interesting for the branches and permission calls: #1039

[lafriks]

I don't think CI failure is related to this PR as there is no changes related to database or datasource module

[lafriks]

@anbraten refactored remote method to return struct and simplified membership service