Decide on the role of usernames and display names for first party accounts

We added "display names" to user profiles so that the presentation of third party users' on annotation cards is not subject to the limitations or uniqueness constraints of our usernames.

We later added a form to the website (at https://hypothes.is/account/profile) to allow first party users to configure their display name and we use it in _some_ places: user profile pages and soon reply notification emails.

However, we don't yet use display names for first party accounts in the client or on activity pages and we haven't got a clear consensus about when we should use one or the other or perhaps some combination of the two. This causes confusion for users who expect that by configuring a display name it will appear on their annotations in the client and generally for developers who are not sure what to use in which context.

So what I'm asking for here is to **figure out a more coherent plan for how we present users' identities across Hypothesis**. We can then file follow-up work to actually implement that.