[5.x] Improve authentication & account registering logic

[miguilimzero]

This pull request improves the new account registration handling. The current code has a duplicate code to handle that, and the logic gets slightly confusing.

Here is an example. Let's consider the flag hasCreateAccountOnFirstLoginFeatures is enabled:

Current version:

• User tries auth from /login with social media, but the account already exists, and it's not connected.

  The first code snippet will catch this because of the route match + flag.

• User tries auth from /register with social media but the account already exists, and it's not connected.

  The first code snippet will catch this because of the route match.

• User tries auth from /potato with social media, but the account already exists, and it's not connected.

  The second code snippet will catch this because there was no route match, and the second code snippet only needs the flag enabled.

With the code refactoring, the first code snippet will handle every route case if the "create account from login" flag is enabled (not only login) and the second code snipped is removed. We are preserving the exact same logic here.

[miguilimzero]

@joelbutcher Sorry, but I could not find a relation between this feature flag and the multiple stacks available. I made the following changes:

• Split the specific feature tests into different files.
• Added test with a random route for createAccountOnFirstLogin feature.
• Added failure test for login and a random route for createAccountOnFirstLogin feature not enabled

[joelbutcher]

Released v5.3.2 with this fix included, sorry for the slowness here – thanks for your work!