Description

After some In-app Purchases testing, I noticed some confusion with the information shown by the IAP debug screen. This PR adds a couple of improvements:

• Disable the purchase button while a purchase is in progress. Since iOS takes a few seconds to show any UI, and there was nothing else shown while communicating with WordPress.com servers, it wasn't clear when the process ended. Now it shows an activity indicator instead of the button while making a purchase.
• Update entitlements more consistently. Previously, the screen loaded the current entitlements when loading products, but didn't update correctly when a purchase was made or a subscription expired unless you reloaded products manually. Now it updates in those cases.

Testing instructions

1. Go to Hub Menu > IAP Debug
2. Purchase a monthly subscription. Notice the button is replaced by an activity indicator until the process finishes.
3. Once the subscription purchase is complete, the button should be back and show "Entitled"
4. Go to iOS Settings > App Store > Manage sandbox account and cancel the subscription
5. Wait a few minutes until the subscription expires and go back to the app
6. The button shouldn't show Entitled anymore. If the subscription hasn't expired yet, sending the app to the background and open again should reload the entitlements.

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.