[GITEA] Fix API inconsistencies

- Document the correct content types for Git archives. Add code that
actually sets the correct application type for `.zip` and `.tar.gz`.
- When an action (POST/PUT/DELETE method) was successful, an 204 status
code should be returned instead of status code 200.
- Add and adjust integration testing.
- Resolves #2180
- Resolves #2181

routers/api/v1/admin/user.go

@@ -545,5 +545,5 @@ func RenameUser(ctx *context.APIContext) {
 	}
 
 	log.Trace("User name changed: %s -> %s", oldName, newName)
-	ctx.Status(http.StatusOK)
+	ctx.Status(http.StatusNoContent)
 }