Fix linter errors

[hanzei]

Summary

Fix linter errors caused by golangci-lint update.

Ticket Link

None

[codecov-commenter]

Codecov Report

Base: 65.73% // Head: 65.79% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (352bb6b) compared to base (dbd4089).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
+ Coverage   65.73%   65.79%   +0.06%     
==========================================
  Files           5        5              
  Lines         534      535       +1     
==========================================
+ Hits          351      352       +1     
  Misses        131      131              
  Partials       52       52              

Impacted Files	Coverage Δ
server/client.go	100.00% <100.00%> (ø)

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[hanzei]

The PR fixes all linter issues expect the one discussed in https://community.mattermost.com/core/pl/au4rusy3mtfuik7e3mzosg9bur. Let's fix most of them and tackle the last one separately.