[NT-498] Prevent app freezing with accessibility inspector

[justinswart]

📲 What

Fixes a app-freezing bug in the simulator when adjusting text size using the accessibility inspector in Manage pledge views.

🤔 Why

Although this didn't appear to be happening on device we do often adjust text sizes with the inspector and this affects our development workflow. Also this seemed to point out something problematic in the layout and warranted further investigation.

🛠 How

• Added a guard clause to prevent calculating the thumbnail size when PledgeDescriptionViewController is hidden.

We hide PledgeDescriptionViewController when we are in any of the Manage pledge flows other than Choose another reward. When this view is hidden it's not necessary to try to calculate the thumbnail size.

✅ Acceptance criteria

• Run the app in simulator on master. Navigate to Update pledge on a project that you've backed. Open the accessibility inspector and adjust text sizes, the app should freeze.
• Check out this branch and repeat the steps above, the app should not freeze in any of the Manage pledge views.

[cdolm92]

Awesome! 🛳🛳🛳