Feat/loading indicators part 1

[AaronPlave]

Improve initial data loading indication. Partially addresses #1581 and #832. General approach is to set various array stores
to null in order to detect if the subscription has returned any results at all yet.

Changes:

• DataGrid and super-component loading prop to control the ag-grid loading message
• DataGrid skeleton loading component
• Track initial load of simulation arguments and display loading message
• Scheduling goal and condition loading states within a plan
• Timeline histogram loading state
• Initial span tracking and loading state
• Scheduling loading state in plan and constraints page
• Constraints loading states in plan and constraints page
• Fix usage of simulation dataset id in one store case
• Fix initial navbar jerkiness on page load
• Activity directive loading indication
• Views loading indication
• Users loading indication
• Simulation history and argument loading indication
• Style bug fix for plan loading where several absolutely positioned timeline elements were not relative to the correct parent and were causing overflow after SSR and before UI interactivity

TODO:

• Management of scheduling conditions within a plan is not working for some reason
• Document the other loading states not yet addressed