Replace GOGS with GITEA in variable names

Still use GOGS_WORK_DIR and GOGS_CUSTOM env variables
as a fallback if the equivalent GITEA_* are not set,
warning user about the need for change.

Does not change "gogs" to "gitea" in webhook type name
Because "gogs" hook type is part of the API (routes) and used
in templates...

Closes #87

routers/repo/pull.go

@@ -687,7 +687,7 @@ func CompareAndPullRequestPost(ctx *context.Context, form auth.CreateIssueForm)
 		HeadRepo:     headRepo,
 		BaseRepo:     repo,
 		MergeBase:    prInfo.MergeBase,
-		Type:         models.PULL_REQUEST_GOGS,
+		Type:         models.PULL_REQUEST_GITEA,
 	}
 	// FIXME: check error in the case two people send pull request at almost same time, give nice error prompt
 	// instead of 500.