Introducing express methods constants for the frontend

[wjrosa]

Changes proposed in this Pull Request:

This PR introduces new constants for the express payment methods (Google Pay, Apple Pay, Link and Amazon Pay). And also implements them on the codebase.

Testing instructions

Code review should be enough. Check if the tests are still passing.

---

• 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

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[diegocurbelo]

I like the idea of having constants, but having to use camelCase everywhere seems confusing.

Having the constants with the same prefix and in the same place as the other payment methods constants might lead to the wrong assumption that they behave the same way (can be used without the need of the camelCase wrapper)

[wjrosa]

I like the idea of having constants, but having to use camelCase everywhere seems confusing.

Having the constants with the same prefix and in the same place as the other payment methods constants might lead to the wrong assumption that they behave the same way (can be used without the need of the camelCase wrapper)

You're right, Diego. I have updated the code in dd8fb3e to introduce specific constants for this use case. What do you think?

I want to convert the constants.js file into a new folder with files for each constant type, but I think it is better to leave that for another PR.

[diegocurbelo]

Thanks for the changes @wjrosa.

It looks good, I'm pre-approving this because the conflicts look minor (just a couple of imports).. but happy to re-test this after you solve them if you want.