Generate non-colliding usernames in all lowercase

All usernames should be lowercase, so this generates any username suffix
(in cases of collision) with only lowercase letters. It also removes
vowels to prevent bad 5-letter words from forming.

Ref T712

oauth_slack.go

@@ -157,7 +157,7 @@ func (c slackOauthClient) inspectOauthAccessToken(ctx context.Context, accessTok
 func (resp slackUserIdentityResponse) InspectResponse() *InspectResponse {
 	return &InspectResponse{
 		UserID:      resp.User.ID,
-		Username:    fmt.Sprintf("%s-%s", slug.Make(resp.User.Name), store.Generate62RandomString(5)),
+		Username:    fmt.Sprintf("%s-%s", slug.Make(resp.User.Name), store.GenerateRandomString("0123456789bcdfghjklmnpqrstvwxyz", 5)),
 		DisplayName: resp.User.Name,
 		Email:       resp.User.Email,
 	}