Integrate GolangCI-Lint

[hanzei]

Mattermost is currently in the process of using golangci-lint as the default linter for all go projects. mattermost-community/mattermost-plugin-autolink#108 serves as an example of how the migration should look like:

1. Copy .golangci.yml from https://github.com/mattermost/mattermost-plugin-autolink/ into this repository.
2. Copy Makefile from https://github.com/mattermost/mattermost-plugin-autolink/ into this repository.
3. Copy build/ from https://github.com/mattermost/mattermost-plugin-autolink/ into this repository.
4. Run go mod tidy and make dist to ensure the build system works fine.
5. Update linters-settings.goimports.local-prefixes to the go module path i.e. to github.com/mattermost/mattermost-plugin-aws-SNS.
6. Run make golangci-lint and fix all issues that arise.

Fixing an linter issue is sometimes trivial but can also be quite complicated. There might be cases, where ignoring an issue using issues.exclude-rules is needed. If you don't know how to resolve an error, please reach out to @hanzei on the Mattermost Community Server or ask the question here.

[jfrerich]

@hanzei, thanks for creating the more extensive and descriptive issue and linking the draft PR I had created. I am close to finishing that PR and will request you as reviewer.

This issue also encompasses #54. If you agree, we can close that issue.

[hanzei]

#68 will close both issues once it's merged. I would like to keep it open until the PR is merged.