[web] Fix thread description and color modification in private thread

Summary:
Fix issue where user is unable to modify the thread description and color in their private thread. The thread name is still unmodifiable (following the inclusion of the EDIT_DESCRIPTION and EDIT_COLOR permissions but not the EDIT_THREAD_NAME permission in D1980), but the thread description and color can be modified.

Related Linear issue with full context here.

Test Plan:
Tested on Chrome/Safari and works as expected:

Before:

After:

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D4073