Fix nuxeo-user-avatar text overflow using 3+ part name

nuxeo-user-avatar uses the first character of each space-delimited part of the User's full name, rather than the first character of the First Name and Last Name each. This means that 3 or more initials may be rendered and overflow outside the filled circle.

Steps to Reproduce:

1. Create a user with a full name comprising 3 or more parts, for example:
   First Name: "First Middle"
   Last Name: "Middle Last"
2. Create a Document and navigate to its View tab to observe metadata.

Expected result: nuxeo-user-avatar displays "FL" for the user initials.

Actual result: nuxeo-user-avatar displays "FMML" overflowing outside filled circle.

(See attached picture for visual reference)