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

@@ -30,6 +30,7 @@ Here's a recommended starter `myplugin.html`:
 
 ``` html
 <dom-module id="my-plugin">
+  <template>
     <script>
       Gerrit.install(plugin => {
         'use strict';
@@ -37,6 +38,7 @@ Here's a recommended starter `myplugin.html`:
         // Your code here.
       });
     </script>
+  </template>
 </dom-module>
 ```
 

Documentation/pg-plugin-styling.txt

@@ -23,6 +23,7 @@ Plugins should provide Style Module, for example:
 
 ``` html
   <dom-module id="some-style">
+    <template>
       <style>
         :root {
           --css-mixin-name: {
@@ -30,6 +31,7 @@ Plugins should provide Style Module, for example:
           }
         }
       </style>
+    </template>
   </dom-module>
 ```
 

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

@@ -7,6 +7,7 @@
 </dom-module>
 
 <dom-module id="my-plugin-style">
+  <template>
     <style>
       html {
         --change-metadata-assignee: {
@@ -23,4 +24,5 @@
         }
       }
     </style>
+  </template>
 </dom-module>

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

@@ -7,6 +7,7 @@
 </dom-module>
 
 <dom-module id="myplugin-app-theme">
+  <template>
     <style>
       html {
         --primary-text-color: #F00BAA;
@@ -15,4 +16,5 @@
         --footer-background-color: #F02BAA;
       }
     </style>
+  </template>
 </dom-module>