refactor(communities, wip): Migrate header back to jinja.

We had previously overridden the communities detail page header template
to use it as the render basis for a custom React header. This has had
negative performance impacts. So this commit

- deactivates the rendering of the custom React header;
- rewrites the same header layout as a jinja template
- moves from client-side fallback pattern generation to rely on prior
  back-end generation of fallback pattern images.
- reverts community submenu items to use registered flask-menu items but
  with reordering via a custom jinja filter;
- moves registration of detail page custom js file into the base.html
  template so that it can be placed in a proper javascript block;
- expose the same "manage" button as upstream, but just as a cog button.

Author: monotasker