fix layout bug for long conversation participants
fix layout bug for long conversation participants

when a conversation has two participants with long
usernames, they overflow the message date.

test plan:
  * create two users with long usernames;
  * send a message to those two users in the new
    conversations UI;
  * verify that the message displays in your sent pane's
    left pane and doesn't have a wonky layout;

Change-Id: Icf8e1ba1c6718d61c92fa7df31b2ede381b8fb7d
Reviewed-on: https://gerrit.instructure.com/24636
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Joel Hough <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@instructure.com>
QA-Review: Clare Strong <40d6bb3ed7fef516c73274131d5a837c16de74fd@instructure.com>
Product-Review: Zach Pendleton <514dbb4d54d4f29e454990df7d075678f43f92aa@instructure.com>

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