Add golangci-lint, fix golangci-lint issues

[vladimirdotk]

Summary

• Copied .golangci.yml from https://github.com/mattermost/mattermost-plugin-autolink/ into this repository.
• Copied Makefile from https://github.com/mattermost/mattermost-plugin-autolink/ into this repository.
• Copied build/ from https://github.com/mattermost/mattermost-plugin-autolink/ into this repository.
• Fixed all golangci-lint issues (tested with the 1.31.0 version).

Ticket Link

Fixes #206

[codecov]

Codecov Report

Merging #208 into master will increase coverage by 0.14%.
The diff coverage is 49.50%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   38.97%   39.11%   +0.14%     
==========================================
  Files          16       16              
  Lines        1578     1585       +7     
==========================================
+ Hits          615      620       +5     
- Misses        895      898       +3     
+ Partials       68       67       -1     

Impacted Files	Coverage Δ
server/api.go	0.00% <0.00%> (ø)
server/configuration.go	14.70% <0.00%> (ø)
server/plugin.go	12.63% <15.00%> (ø)
server/command.go	32.77% <18.18%> (-0.10%)	⬇️
server/subscriptions.go	44.44% <25.00%> (-0.62%)	⬇️
server/utils.go	36.84% <33.33%> (ø)
server/webhook/issue.go	81.42% <75.00%> (-0.39%)	⬇️
server/subscription/subscription.go	100.00% <100.00%> (ø)
server/webhook/merge_request.go	88.00% <100.00%> (+0.67%)	⬆️
server/webhook/pipeline.go	85.96% <100.00%> (ø)

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[hanzei]

Great work 👍 Thanks for going through all of these files. A few questions/comments below

[hanzei]

@vladimirdotk One small thing: Would you mind not force pushing to your branch? It makes it harder for the review to only review the changes that happen since the last review. Commits will be squashed anyway on merge. You can find more about the reasoing at https://developers.mattermost.com/blog/submitting-great-prs/#4-avoid-force-pushing.

[hanzei]

Nice 🚀

[iomodo]

LGTM! Thanks @vladimirdotk

[hanzei]

@DHaussermann Is just requires a general smoke test

[DHaussermann]

Tested and passed

• Build the plugin. No issues woth go lint
• Deployed and briefly smoke tested several slash commands ad well as wenhook delivery and notifications.
• No issues found.

LGTM!
Thanks @vladimirdotk for this PR!