Improvement to OLButton loading labels (#28659)

* Create eslint rule for requiring loadingLabel prop when isLoading is specified on OLButton

* Add `loadingLabel` props for OLButton components with `isLoading`

* Clarify loading label and button loading state

GitOrigin-RevId: 89279d5b4c346f9c3b67a59d0db822a2ff04314a

services/web/test/frontend/features/project-list/components/notifications.test.tsx

@@ -884,7 +884,7 @@ describe('<UserNotifications />', function () {
         screen.getByRole('button', { name: 'Send confirmation code' })
       )
 
-      await waitForElementToBeRemoved(() => screen.getByText(/loading/i))
+      await waitForElementToBeRemoved(() => screen.getByText(/sending/i))
       screen.getByText(/Enter the 6-digit code sent to foo@overleaf.com/i)
       expect(sendReconfirmationMock.callHistory.called()).to.be.true
       fireEvent.click(

services/web/test/frontend/features/settings/components/emails/emails-section-add-new-email.test.tsx

@@ -220,7 +220,7 @@ describe('<EmailsSection />', function () {
 
     await waitForElementToBeRemoved(() =>
       screen.getByRole('button', {
-        name: 'Loading',
+        name: /adding/i,
       })
     )