Move <style> inside <template> in all plugin files

This is a required step for Polymer 2 compatibility:
https://polymer-library.polymer-project.org/2.0/docs/upgrade#remove-deprecated-patterns-in-dom-module

Bug: Issue 11107
Change-Id: I3fd017326d412817f6650dcffe53f2b022a66e64

Documentation/pg-plugin-dev.txt

@@ -30,13 +30,15 @@ Here's a recommended starter `myplugin.html`:
 
 ``` html
 <dom-module id="my-plugin">
-  <script>
-    Gerrit.install(plugin => {
-      'use strict';
+  <template>
+    <script>
+      Gerrit.install(plugin => {
+        'use strict';
 
-      // Your code here.
-    });
-  </script>
+        // Your code here.
+      });
+    </script>
+  </template>
 </dom-module>
 ```
 

Documentation/pg-plugin-styling.txt

@@ -23,13 +23,15 @@ Plugins should provide Style Module, for example:
 
 ``` html
   <dom-module id="some-style">
-    <style>
-      :root {
-        --css-mixin-name: {
-          property: value;
+    <template>
+      <style>
+        :root {
+          --css-mixin-name: {
+            property: value;
+          }
         }
-      }
-    </style>
+      </style>
+    </template>
   </dom-module>
 ```
 

polygerrit-ui/app/elements/change/gr-change-metadata/test/plugin.html

@@ -7,20 +7,22 @@
 </dom-module>
 
 <dom-module id="my-plugin-style">
-  <style>
-    html {
-      --change-metadata-assignee: {
-        display: none;
+  <template>
+    <style>
+      html {
+        --change-metadata-assignee: {
+          display: none;
+        }
+        --change-metadata-label-status: {
+          display: none;
+        }
+        --change-metadata-strategy: {
+          display: none;
+        }
+        --change-metadata-topic: {
+          display: none;
+        }
       }
-      --change-metadata-label-status: {
-        display: none;
-      }
-      --change-metadata-strategy: {
-        display: none;
-      }
-      --change-metadata-topic: {
-        display: none;
-      }
-    }
-  </style>
+    </style>
+  </template>
 </dom-module>

polygerrit-ui/app/elements/test/plugin.html

@@ -7,12 +7,14 @@
 </dom-module>
 
 <dom-module id="myplugin-app-theme">
-  <style>
-    html {
-      --primary-text-color: #F00BAA;
-      --header-background-color: #F01BAA;
-      --header-title-content: "MyGerrit";
-      --footer-background-color: #F02BAA;
-    }
-  </style>
+  <template>
+    <style>
+      html {
+        --primary-text-color: #F00BAA;
+        --header-background-color: #F01BAA;
+        --header-title-content: "MyGerrit";
+        --footer-background-color: #F02BAA;
+      }
+    </style>
+  </template>
 </dom-module>