Mattermost-server: [Help Wanted] [MM-11293] Add server Makefile target to run Go static checker

[esethna]

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Notes: Jira ticket

This ticket entails:

1. Add a Makefile target to add static code checking to the server repository
2. Fix all the issues reported by the static checker

A follow-up ticket will be used to add this to the CI pipeline once it's working and all issues are fixed.

Preferably use a Go-based checker so that the Makefile can install it as needed.

[DSchalla]

I'll take it.