[Sentry] Create a transaction when loading individual notes

[mplorentz]

Create a Sentry transaction whenever we attempt to load an individual note. Mainly I think we do this when displaying a repost or a quoted note. Let's call it Note.download

Additionally, create a widget in our Sentry dashboard that we can use to see how long on average we are taking to load a note. The goal here is mainly to understand if we have shipped a regression with content loading.