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/directives/namespace-input.html

@@ -0,0 +1,5 @@
+<span class="namespace-input-element">
+  <input type="text" class="form-control" placeholder="{{ namespaceTitle }}" ng-model="binding"
+         required autofocus ng-pattern="usernamePattern"
+         name="namespaceField">
+</span>