Have "limit=1" tokenizers replace tokens instead of disabling "Browse"

Summary:
Fixes T9984. When a tokenizer only allows one selection (like "Task Owner:" or "Land Onto Branch:"), keep the browse button active but have it replace values.

Also, have "Create Subtask" default to the system default status, so subtasks of closed tasks are not also closed.

Test Plan:

• Browsed an empty limit=1 tokenizer.
• Replaced a full limit=1 tokenizer.
• Browsed an empty no-limit tokenizer.
• Browsed more tokens into the no-limit tokenizer.
• Typed some tokens normally.
• Created a subtask of a closed task.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9984

Differential Revision: https://secure.phabricator.com/D14785