[keyserver] JSON.parse() stringified avatar in fetchLoggedInUserInfo

Summary:
We want to JSON.parse() the stringified avatar from the database before including in LoggedInUserInfo and returning.

Noticed that on user login avatar was a string instead of an object of type ClientAvatar.

The mysql2 node package conveniently "auto parsed" stringified JSON when providing the result set with MySQL, but unfortunately doesn't for MariaDB (https://github.com/sidorares/node-mysql2/issues/1287)... so we need to JSON.parse(...) ourself.

Test Plan: Logged in and avatar was object of type ClientAvatar as expected:

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D7157