Refactor user dropdown logic into reusable component

[mikkonie]

Sequel to #1380, also needed for #1366.

Similar to the sidebar, we want to be able to provide the user dropdown links also in an Ajax view for possible client side UIs. And similar to the sidebar, the logic for the dropdown is contained in the context processor, template and template tags.

So, you guessed it: we need to refactor the user dropdown in a similar way to the sidebar.

Spec

• Add get_user_links() method to AppLinkContent helper created in Refactor sidebar logic into reusable component #1380
  • request should probably be the only arg required
  • Return list of dicts similar to the method in Refactor sidebar logic into reusable component #1380
• Move logic from _site_titlebar_dropdown.html and related template tags into new method
• Deprecate site_app_processor()
• Add wrapper template tag to retrieve list into _site_titlebar_dropdown.html
  • Call it and render the dropdown links in the template
• Add tests for new helper class
• Add/update UI tests
  • I'm not sure if we have UI tests for the menu? If not, a new class should be added next to TestProjectSidebar. Testing with both regular user and admin
• Remove unused template tags (if any)

Tasks

• Add helper class
• Move user dropdown logic into class
• Deprecate site_app_processor
• Update tests
• Remove unused template tags (if any)

[mikkonie]

Done by @gromdimon and merged.