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>

static/directives/signup-form.html

@@ -9,7 +9,16 @@
 
   <form class="form-signup" name="signupForm" ng-submit="register()" ng-show="!awaitingConfirmation && !registering">
     <label for="username">Username:</label>
-    <input type="text" class="form-control" placeholder="Requested username" name="username" ng-model="newUser.username" autofocus required ng-pattern="/^[a-z0-9_]{4,30}$/">
+    <span class="namespace-input" binding="newUser.username" is-back-incompat="isBackIncompat" namespace-title="Requested username"></span>
+
+    <div class="expandable" ng-class="{'expanded': isBackIncompat || (!signupForm.namespaceField.$error.required && signupForm.namespaceField.$invalid)}">
+      <div class="co-alert co-alert-warning thin" ng-show="isBackIncompat">
+        Usernames with dots or dashes are incompatible with Docker verion 1.8 or older
+      </div>
+      <div class="co-alert co-alert-danger thin" ng-show="!signupForm.namespaceField.$error.required && signupForm.namespaceField.$invalid">
+        Usernames must be alphanumeric and be at least four characters in length
+      </div>
+    </div>
 
     <label for="email">E-mail address:</label>
     <input type="email" class="form-control" placeholder="Your email address" name="email" ng-model="newUser.email" required>
@@ -20,6 +29,8 @@
     <input type="password" class="form-control" placeholder="Verify your password" ng-model="newUser.repeatPassword"
            match="newUser.password" required
            ng-pattern="/^.{8,}$/">
+
+
     <button id="signupButton"
             class="btn btn-primary btn-block landing-signup-button" ng-disabled="signupForm.$invalid" type="submit"
             analytics-on analytics-event="register">