gui: Improve revert/override/delete warning modals (#7847)

1. Change each modal title text to match the action that is being
   executed (i.e. "Revert" to "Revert Local Additions", "Override" to
   "Override Changes", "Delete" to "Delete Unexpected Items").
2. Change the icons to match the icons used by each action (i.e. arrow-
   circle-down for Revert, arrow-circle-up for Override). Replace the
   broken lock icon for Delete with minus-circle.
3. Rearrange the order in the modal HTML code to simplify it a little.

gui/default/index.html

@@ -557,7 +557,7 @@
                     <span class="fa fa-arrow-circle-down"></span>&nbsp;<span translate>Revert Local Changes</span>
                   </button>
                   <button type="button" class="btn btn-sm btn-danger pull-left" ng-click="revertOverrideConfirmationModal('deleteEnc', folder.id)" ng-if="hasReceiveEncryptedItems(folder)">
-                    <span class="fa fa-arrow-lock"></span>&nbsp;<span translate>Delete Unexpected Items</span>
+                    <span class="fa fa-minus-circle"></span>&nbsp;<span translate>Delete Unexpected Items</span>
                   </button>
                   <span class="pull-right">
                     <button ng-if="!folder.paused" type="button" class="btn btn-sm btn-default" ng-click="setFolderPause(folder.id, true)">