Accessibility fixes for auiplugin-6.0.6

Problem Definition

Currently there are a few accessibility issues with the templates for this plugin, for example:

• page.soy
  HTML Lang attribute not being carried forward
• forms.less
  Add css classes to adjust sidebar for anonymous browsing when space tools button is not available. > (happens in space-ia)
  Space Overview convert Labels to span's.
• avatar.soy
  Add blank alt text to Avatars if one is not present.
• dropdown2.soy
  Remove role="presentation" from space tools menu items
• labels.soy
  Add better assistive text to label items
• sidebar.soy
  Add aria-label for sidebar expand toggle