all: Add receive only folder type (#5027)

Adds a receive only folder type that does not send changes, and where the user can optionally revert local changes. Also changes some of the icons to make the three folder types distinguishable.

gui/default/syncthing/folder/editFolderModalView.html

@@ -162,8 +162,10 @@
               <select class="form-control" ng-model="currentFolder.type">
                 <option value="sendreceive" translate>Send &amp; Receive</option>
                 <option value="sendonly" translate>Send Only</option>
+                <option value="receiveonly" translate>Receive Only</option>
               </select>
               <p ng-if="currentFolder.type == 'sendonly'" translate class="help-block">Files are protected from changes made on other devices, but changes made on this device will be sent to the rest of the cluster.</p>
+              <p ng-if="currentFolder.type == 'receiveonly'" translate class="help-block">Files are synchronized from the cluster, but any changes made locally will not be sent to other devices.</p>
             </div>
             <div class="col-md-6 form-group">
               <label translate>File Pull Order</label>