Add loading state to volunteer-headshot component

Problem

The volunteer-headshot should have some loading states for async data.

Subtasks

• Add a light gray background for the image as a placeholder, until loaded (keeping in mind that a user may have a transparent PNG, so the background shouldn’t remain gray)
• Add a light gray rectangular area for the user’s name as a placeholder, until loaded
• Add a light gray rectangular area for the user’s role as a placeholder, until loaded

[alistermada]

Is this okay? This was shot while using the mirage fake API with 4 sample users.

[joshsmith]

Opening a PR is a good first step so we can start the conversation around it.