Improvement of buttons on TorrentViewer

- Removed redundant rules, adding "primaryButton" class
- Removed float:right by aligning the buttons on the js file

Fixes #6735

Auditors:

Test Plan:
1. Visit https://codepen.io/ferossity/full/qaezaB/
2. Open any link on the page in a new tab
3. Make sure there is a gap between the buttons on the header

js/webtorrent/components/torrentViewer.js

@@ -45,15 +45,15 @@ class TorrentViewer extends React.Component {
         <div className='siteDetailsPageHeader'>
           {titleElem}
           <div className='headerActions'>
-            <Button
-              l10nId='saveTorrentFile'
-              className='whiteButton saveTorrentFile'
-              onClick={() => dispatch('saveTorrentFile')} />
             <Button
               l10nId={mainButtonId}
-              className='whiteButton mainButton'
+              className='primaryButton mainButton'
               disabled={!!torrent}
               onClick={() => dispatch('start')} />
+            <Button
+              l10nId='saveTorrentFile'
+              className='whiteButton saveTorrentFile'
+              onClick={() => dispatch('saveTorrentFile')} />
           </div>
         </div>
 

less/webtorrent.less

@@ -10,16 +10,7 @@
   min-width: 704px;
 
   .siteDetailsPageHeader {
-    .headerActions > * {
-      float: right;
-    }
     .mainButton {
-      background: linear-gradient(@braveLightOrange, @braveOrange);
-      border: 2px solid transparent;
-      border-top: 2px solid @braveLightOrange;
-      border-bottom: 2px solid @braveOrange;
-      box-shadow: @buttonShadow;
-      color: white;
       font-weight: 400;
     }
   }