Add display of the "Reviewers" field to the pull request creation event template

[San4es]

At the moment, the "Assignees" field displays:

mattermost-plugin-github/server/plugin/template.go

Lines 210 to 214 in 827e881

	template.Must(masterTemplate.New("assignee").Funcs(funcMap).Parse(`
	{{- if .Assignees }}
	Assignees: {{range $i, $el := .Assignees -}} {{- if $i}}, {{end}}{{template "user" $el}}{{end -}}
	{{- end -}}
	`))

I suggest to add the "Reviewers" field as well to notify PR reviewers in the pulls_created event via their mention.