Allow multiple modules to be laid out on a page

[ramnathv]

In many situations, a page consists of standalone submodules that are assembled as a combination of tabs and rows. This can be made easier by extending the API of satin.

- text: Item 1
   menu:
     - text: Sub Item 1
       module:
          tabs:
          - text: Tab 1
            module:
             tab1:
               id: tab1
          - text: Tab 2
            module: tab2

In this case module_tabs is a satin module that accepts a list of modules and turns them into tabs on a page. module_rows would turn a list of modules into rows. The advantage of this approach is consistency and the ability for users to define custom higher-level modules to aggregate submodules.

It does introduce an extra layer of nesting which might be annoying, but I think consistency scores higher than the annoyance introduced by a little nesting.

[ramnathv]

The alternative is to support a flatter API.

- text: Engagement
  rows:
    - module:
        panel_metric:
          id: product_engagement_active_users
          plot_type: line
    - module:
        panel_metric:
          id: product_engagement_time_spent
          plot_type: line
        
        
- text: Engagement
  tabs:
    - text: Active Users
      module: 
        panel_metric:
          id: product_engagement_active_users
          plot_type: line
    - text: Time Spent
      module:
        panel_metric:
          id: product_engagement_time_spent
          plot_type: line

This reduces the extent of nesting and also makes the intent clear that the modules are to be laid out as tabs (or rows). I need to think about how it might reduce flexibility in comparison to the approach outlined previously.