* model/config: add required server config field to manifest * plugin/helpers: add check required server configuration method * plugin/helpers: code polish * model/manifest: add documentation * Update plugin/helpers_config.go Co-Authored-By: Ben Schumacher <ben.schumacher@mattermost.com> * plugin/helpers_config: remove stagnant line * plugin/helpers: update to v5 * plugin/helpers_config: add license Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com> Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com> Co-authored-by: mattermod <mattermod@users.noreply.github.com>