Invalid url validation on admin->server config Register url field

Description of problem:
The server configuration page in Administration has url validation on the Register url field - so the previously accepted /zanata/register is now rejected as invalid

Version-Release number of selected component (if applicable):
3.6-snapshot

How reproducible:
Easy always

Steps to Reproduce:
1. sign in as admin, go to Administration, Server config
2. Enter /zanata/register into the Register URL field (as indicated by the placeholder)
3. Press Enter or Save

Actual results:
Entry rejected

Expected results:
Entry accepted

Additional info: