expose first name / last name in reply objects

[redshiftzero]

In freedomofpress/securedrop-sdk#92 we added support for first name / last name for the API endpoint that gets details about the current user.

We also have first names and last names exposed in the API for replies: freedomofpress/securedrop#4459. We expose first name and last name on replies such that we can create the user badge based on their initials in securedrop-client - so we'll need to support first name / last name also on the reply objects in the SDK.

Note that #411 is not blocked by this issue since we do have support for the current user.