Disable tokenizer "Browse" button once tokenizer is full

Summary: Ref T7858. Don't let users add multiple values to single-value tokenizers by using the "Browse" button.

Test Plan:

• Added a token, browse button was disabled.
• Removed the token, browse button was enabled again.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7858

Differential Revision: https://secure.phabricator.com/D14738