[Sentry] Track loading of profile images

[mplorentz]

Create a Sentry transaction whenever we display a profile photo in the UI. Let's call it ProfileImage.load.

This transaction should begin when a view like AvatarView is initialized and complete when a profile image is displayed, excluding the placeholder image. Ideally we would also track spans to understand what amount of time is spent getting the image URL from relays, downloading the image over HTTP, and resizing it to be displayed in the UI. If these spans are too complex to implement lets discuss alternatives.

Additionally, create a widget in our Sentry dashboard that we can use to see how long these transactions take on average. The goal here is to understand how long our users are typically waiting for the profile images to load, and detect regressions.