Systems Listing react component improvements

[jchuahtacc]

• Rename the component to SystemList and put it in a subfolder called Systems. (The Systems object from tapis-typescript generates too many naming conflicts, and we can stash any Systems related UI components into a group that way. Like SystemDetail, SystemsList, SystemCredential)
• Add an onSelect callback property
• Support firing of onSelect so that the component can allow a user to select a system and a parent component can receive the selection event
• Add loading indicator when systems are being retrieved
• Clean up design, possibly using _common components to represent the list with selectable items.

[jchuahtacc]

After discussion with @Strmiska:

• The SystemsList component could be patterned off of the CEPv2 system selector in Data Files:
  
• multi-select is not important since nothing takes a set of systems as input