Add scrolling functionality to feature flag page (#5994)

* Motivation for features / changes
In hosted services we have too many flags to fit on many screens. Before
this change that meant the bottom flags and the reset button were cut
off. Now we can scroll down to see all flags and the reset button.

* Screenshots of UI changes
![2022-10-24_16-43-09
(1)](https://user-images.githubusercontent.com/8672809/197650840-f5293cbf-dfdb-40b9-bc14-c64336d57102.gif)

Author: JamesHollyer

tensorboard/webapp/feature_flag/views/feature_flag_page_component.ng.html

@@ -15,7 +15,7 @@
 limitations under the License.
 -->
 
-<div>
+<div class="scrolling-page">
   <div class="message">
     <h2 class="warning">WARNING: EXPERIMENTAL FEATURES AHEAD!</h2>
     By enabling these features, you could put the application in an unusable

tensorboard/webapp/feature_flag/views/feature_flag_page_component.scss

@@ -22,6 +22,11 @@ limitations under the License.
   margin-bottom: 16px;
 }
 
+.scolling-page {
+  height: 90vh;
+  overflow-y: scroll;
+}
+
 .feature-flag-table {
   width: 100%;
 }