Better loading UI

[parkerabercrombie]

Checked for duplicates

No - I haven't checked

Alternatives considered

Yes - and alternatives don't suffice

Related problems

When there is lots of data in the system the Aerie UI feels slow and needs to tell the user when the data is still loading.

Describe the feature request

We have lots of models and plans on our dev and ops venues, and many actions in the UI can take several seconds to complete. Most parts of the UI do not distinguish "no data" from "still loading the data". This makes it look like data is missing or Aerie is broken for a few seconds before the data finally appears.

I think this applies to almost every part of the UI, but the areas where I notice it the most are simulation arguments (select the simulation sidebar, and the arguments section is empty for several seconds until the args load), activity directive parameters, and views.

[AaronPlave]

List of UI components/areas that need loading indicators:

• Models
• Plans
• Activity directives
• Activity directive parameters (might be solved by using the pre-fetched activity arg defaults from Advanced Timeline Filtering #1535 but needs separate PR)
• Spans
• Constraints
• Dictionaries
• Parcels
• Expansion
• Scheduling Goals and Conditions
• Sequences
• External Sources
• Tags
• Views
• Simulations
• Simulation arguments
• Simulation Events
• Errors

This list will be updated as indicators are added and as additional areas are found.