[mgt-person] render placeholder while loading/fetching data

[nmetulev]

Description

The person component does not currently render anything while it is loading. Update the person component to always allocate the needed space when loaded.

Rationale

As the person object is loading, because the space is not allocated, the page moves around when the person component finally loads. When there are a lot of people objects on the page, this can cause the page content to move quite a bit.

Preferred Solution

Rendering a placeholder UI is a good way to handle this - the user will see something loading