Device flow token code exchange (#2)

* Added /device/token handler with associated business logic and storage tests.

Perform user code exchange, flag the device code as complete.

Moved device handler code into its own file for cleanliness.  Cleanup

* Removed PKCE code

* Rate limiting for /device/token endpoint based on ietf standards

* Configurable Device expiry

Signed-off-by: justin-slowik <justin.slowik@thermofisher.com>

web/templates/device.html

@@ -0,0 +1,23 @@
+{{ template "header.html" . }}
+
+<div class="theme-panel">
+  <h2 class="theme-heading">Enter User Code</h2>
+  <form method="post" action="{{ .PostURL }}" method="get">
+    <div class="theme-form-row">
+      {{ if( .UserCode  )}}
+      <input tabindex="2" required id="user_code" name="user_code" type="text" class="theme-form-input" autocomplete="off" value="{{.UserCode}}" {{ if .Invalid }} autofocus {{ end }}/>
+      {{ else }}
+      <input tabindex="2" required id="user_code" name="user_code" type="text" class="theme-form-input" placeholder="XXXX-XXXX" autocomplete="off"  {{ if .Invalid }} autofocus {{ end }}/>
+      {{ end }}
+    </div>
+
+    {{ if .Invalid }}
+    <div id="login-error" class="dex-error-box">
+      Invalid or Expired User Code
+    </div>
+    {{ end }}
+    <button tabindex="3" id="submit-login" type="submit" class="dex-btn theme-btn--primary">Submit</button>
+  </form>
+</div>
+
+{{ template "footer.html" . }}

web/templates/device_success.html

@@ -0,0 +1,8 @@
+{{ template "header.html" . }}
+
+<div class="theme-panel">
+  <h2 class="theme-heading">Login Successful for {{ .ClientName }}</h2>
+  <p>Return to your device to continue</p>
+</div>
+
+{{ template "footer.html" . }}