gui: Add debug tab to settings (ref #2644)

Just because there are a ton of people struggling to set env vars.
Perhaps this should live in advanced settings, and perhaps we should have a button to view the log.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4604
LGTM: calmh, imsodin

gui/default/syncthing/settings/settingsModalView.html

@@ -150,6 +150,7 @@
             </div>
           </div>
         </div>
+
         <div id="settings-connections" class="tab-pane">
           <div class="form-group">
             <label translate for="ListenAddressesStr">Sync Protocol Listen Addresses</label>&emsp;<a href="https://docs.syncthing.net/users/config.html#listen-addresses" target="_blank"><span class="fa fa-fw fa-book"></span>&nbsp;<span translate>Help</span></a>
@@ -221,11 +222,12 @@
                 <label translate for="GlobalAnnServersStr">Global Discovery Servers</label>
                 <input ng-disabled="!tmpOptions.globalAnnounceEnabled" id="GlobalAnnServersStr" class="form-control" type="text" ng-model="tmpOptions._globalAnnounceServersStr"/>
               </div>
-            <div>
+            </div>
             <div class="col-md-6">
             </div>
           </div>
         </div>
+
       </div>
     </form>
   </div>