People: Set maxLength of invitees to 10

[ebinnion]

Closes #3615

Adds a maxLength prop to TokenField and sets the maxLength of invitees to 10.

To test:

• Checkout update/token-field-add-max-length branch
• Go to /people/new/$site where $site is a WP.com site
• Attempt to enter more than 10 invitees
• You should only be able to enter 10
• Remove a token by clicking the "x"
• Now you should be able to add another token

cc @lezama for review

[lezama]

works as described :)

🚢