messages: custom content described in customcontent.go, message tests

customcontent.go constains a structure with all the custom content,
methods for getting display names, subjects, etc., and a list of
variables, conditionals, and placeholder values. Tests for constructX
methods included in email_test.go, and all jfa-go tests can be run with
make INTERNAL=off test.

updater.go

@@ -130,7 +130,7 @@ type Updater struct {
 	binary                                     string
 }
 
-func newUpdater(buildroneURL, namespace, repo, version, commit, buildType string) *Updater {
+func NewUpdater(buildroneURL, namespace, repo, version, commit, buildType string) *Updater {
 	// fmt.Printf(`Updater intializing with "%s", "%s", "%s", "%s", "%s", "%s"\n`, buildroneURL, namespace, repo, version, commit, buildType)
 	bType := off
 	tag := ""