Show how long the user has been a member on user hover cards

[braid-sync]

A task for new Braid contributors.

Goal:
On user-hover-cards (cards that show up when hovering over a user mention), show how long the user has been a member.

Notes:

• the server will need to include this info in the exist user data that it sends to the client (we'll leave it to you to figure where that is)
• we currently don't explicitly store a timestamp for when a user joins a group, but, it can be figured out by getting the :db/txInstant of when the :group/user datom was created. Some example code:

(d/q
 '[:find [?inst ...]
 :in $ ?user-id ?group-id
 :where
 [?u :user/id ?user-id]
 [?g :group/id ?group-id]
 [?g :group/user ?t ?tx false]
 [?tx :db/txInstant ?inst]]
 (d/history (db/db))
 some-user-id
 some-group-id)

[jamesnvc]

Closed by #207