[web] Increase default size of thread description textarea in ThreadSettingsGeneralTab

Summary:
Increased the default size of the textarea that contains the thread description in ThreadSettingsGeneralTab to display 3 rows. This is because the old size did not fully display the default thread description ("This is your private thread, where you can set reminders and jot notes in private!") with the last line cut off.

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/D4104