Better handling of namespace validation to fix a number of issues

- Fixes a bug which allows for underscores at the beginning of namespaces: Fixes #1849
- Allows dots and dashes for newer Docker clients: Fixes #1188
- Has the UI display better messaging associated with namespace entry

static/js/app.js

@@ -1,6 +1,6 @@
 var TEAM_PATTERN = '^[a-z][a-z0-9]+$';
 var ROBOT_PATTERN = '^[a-z][a-z0-9_]{3,29}$';
-var USER_PATTERN = '^[a-z0-9_]{4,30}$';
+var USERNAME_PATTERN = '^([a-z0-9]+(?:[._-][a-z0-9]+)*){4,30}$';
 
 // Define the pages module.
 quayPages = angular.module('quayPages', [], function(){});