Detach styling from semantic HTML header tags

We want to use <h1-6> header tags for semantic structuring of all Gerrit
pages in order to increase accessibility. Using such a header anywhere
in the app should by itself not have any influence on the appearance. So
we are removing the basic css rules for h1, h2, h3 from shared styles.

In order to make this change a visual no-op we are applying heading-$n
classes to all existing header tags.

Once this change is merged you can change the n of any <h$n> tag without
affecting how Gerrit looks to users. That will *only* change the
structure. And changing the n of a class="heading-$n" attribute will
*only* change the look, not the structure.

Change-Id: Ic5e267208d164ba6ecd49992a63fa6b056e42a22

polygerrit-ui/app/elements/admin/gr-group-members/gr-group-members_html.js

@@ -63,9 +63,9 @@ export const htmlTemplate = html`
       Loading...
     </div>
     <div id="loadedContent" class$="[[_computeLoadingClass(_loading)]]">
-      <h1 id="Title">[[_groupName]]</h1>
+      <h1 id="Title" class="heading-1">[[_groupName]]</h1>
       <div id="form">
-        <h3 id="members">Members</h3>
+        <h3 id="members" class="heading-3">Members</h3>
         <fieldset>
           <span class="value">
             <gr-autocomplete
@@ -112,7 +112,7 @@ export const htmlTemplate = html`
             </tbody>
           </table>
         </fieldset>
-        <h3 id="includedGroups">Included Groups</h3>
+        <h3 id="includedGroups" class="heading-3">Included Groups</h3>
         <fieldset>
           <span class="value">
             <gr-autocomplete