[PS-1307] Desktop: Change box-headers from <div> to semantic headings (#3313)

* Change box-headers from generic `<div>` to semantic headings, tweak heading styles

Essentially, a late port to desktop of aspects from https://github.com/bitwarden/clients/pull/2223 (which I hadn't realised at the time also affected the desktop app)

* Change box headers in modals to `<h1>`s

* Fix/normalise modals

* Harmonise modal dialog headings, use `aria-labelledby`

apps/desktop/src/app/components/password-reprompt.component.html

@@ -1,9 +1,11 @@
-<div class="modal fade" role="dialog" aria-modal="true">
+<div class="modal fade" role="dialog" aria-modal="true" aria-labelledby="passwordConfirmationTitle">
   <div class="modal-dialog modal-dialog-scrollable" role="document">
     <form class="modal-content" #form (ngSubmit)="submit()">
       <div class="modal-body">
         <div class="box">
-          <div class="box-header">{{ "passwordConfirmation" | i18n }}</div>
+          <h1 class="box-header" id="passwordConfirmationTitle">
+            {{ "passwordConfirmation" | i18n }}
+          </h1>
           <div class="box-content">
             <div class="box-content-row box-content-row-flex" appBoxRow>
               <div class="row-main">