allow retention tag names to be updated

Also converted some flash-message-based error handling to use TG
validators.

Bug: 1012389
Change-Id: Ib9810928b718270b22874dc889599b3adae05b90
allow admins to edit retention tag expiry time

Bug: 1012389
Change-Id: I926bde64fa9039e2c7f8da0cfde11e60be169b56
test_retentiontag: ported Selenium tests to WebDriver

Change-Id: I68bfc66f39033b9c335693086219fc6f35f8f6ff