DX-2042: Ensure that includes are used where possible

[asbjornu]

In this PR, some effort has been made to create an RSpec which ensures that where we have includes, they are used. The spec works as following:

1. Find all field and event includes and map them to their corresponding JSON and JavaScript fields.
2. For each line in every .md file in the portal, find all lines that look like a table row.
3. For each table row line, check whether a field from step 1 is found.
4. If a field is found, see whether the line contains the corresponding include file.
5. If an include is not found on the line, create a GitHub code review comment to the pull request.
6. If creating a code review comment fails, make a regular RSpec assertion, failing the test suite.

The spec mostly works, but needs tweaking to exclude false positives where there is a match on the field but the corresponding include is for a similarly named field in another request or response model. The creation of the GitHub code review also fails, due to the following error:

POST https://api.github.com/repos/SwedbankPay/developer.swedbankpay.com/pulls/1711/reviews: 422 - Unprocessable Entity
Error summary: Pull request review thread position is invalid and Pull request review thread diff hunk can't be blank // See: https://docs.github.com/rest/reference/pulls#create-a-review-for-a-pull-request