text: Title: [Minor issue] [App Icon rendering] Using the macro navbar_block
  Body: Hello Team,
  
  Thank you for this amazing Framework. Just to notify a minor issue that can be solved quicly (sorry for not using the usual issue template....)
  
  The code inside the macro navbar_block have some missing html attributes compared to the navbar.html file 
  This is related to the app_icon rendering.
  
  
  **Line 17 : `<img src="{{appbuilder.app_icon}}" height="100%" width="auto">`**
  from source ->
  https://github.com/dpgaspar/Flask-AppBuilder/blob/98b1be8b3390cd592dc20f215062e55d27e08eec/flask_appbuilder/templates/appbuilder/navbar.html
  
  and
  **Line 93 : `<img src="{{appbuilder.app_icon}}" >`**
  from source -> https://github.com/dpgaspar/Flask-AppBuilder/blob/1e900bba85452de6d988f7da191f9a26fec62226/flask_appbuilder/templates/appbuilder/baselib.html
   
  As result, the app_icon is rendered in differrent ways depending on if we use the macro or if we extends direclty from baselayout.html. 
  
  Thank you all again for your great work.
  
  