[web] Memoize loginButtonContent and saveButtonContent

Summary: Noticed these could be memoized.

Test Plan: Made sure everything looked as expected on the LogInForm modal and ThreadSettingsGeneralTab save button. Loading indicators showed up correctly, disappeared after submit was complete, etc.

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D4930