Dynamically fetch OAuth URLs when needed

[malithsen]

Relates to #3692

Changes proposed in this Pull Request:

This introduces a minor improvement to the Stripe Settings page load by avoiding Stripe OAuth API calls when not required.

Stripe API calls are costly and making those calls on page load introduces ~2s page load time when testing locally. Currently there are 2 cases where we make API calls to Stripe.

1. To load OAuth URLs
2. To load Stripe account data.

This PR only addresses the 1st case.

There are three states where OAuth URL is needed.

1. For new merchants who have not yet connected their Stripe account.
2. For merchants who need to be reconnected using OAuth (Introducing the Re-Connect Promotional Surface #3322)
3. When merchants want to reconnect the Stripe account via "Configure connection" modal in Settings.

With this PR, we only make the external Stripe API calls to get OAuth URLs if the store is in 1st or 2nd states.
For 3rd state, we delay the URL fetch until the modal is opened.

In my local tests, this saves ~400ms when loading the settings page.

For comparison, following is how the cachegrind call tree looks like for curl_exec before and after for a connected merchant.

Before

After

Testing instructions

We mainly want to do regression tests around the three scenarios where the OAuth URL is used.

1. Go through onboarding as a new merchant and make sure you're able to connect the stripe account.
2. Re-test Introducing the Re-Connect Promotional Surface #3322 and make sure "Configure connection" button in the promotional surface is still functional.
3. As a connected merchant, go to Settings -> Configure connection and make sure the "Create or connect account" button works

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• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[diegocurbelo]

Great job @malithsen, I really like this solution 👏🏼.

What do you think about moving the loading spinner inside the blue button (and disabling it)... the reconfigure button does not need the urls so it would work even if the oauth urls fail to load.

[malithsen]

What do you think about moving the loading spinner inside the blue button (and disabling it)... the reconfigure button does not need the urls so it would work even if the oauth urls fail to load.

Great suggestion! Thanks @diegocurbelo updated in 7fda578