feat: allow overriding showLoginPage config via request header

[whipermr5]

Problem

• Mockpass is very suitable as a mock Singpass server for writing E2E tests against, except for the HTML login page
• We can set the SHOW_LOGIN_PAGE environment variable to false to overcome this, but then it becomes less suitable for manual testing
• We then need to maintain two separate configurations / instances of Mockpass, one for automated E2E tests, and another for manual tests

Solution

• Allow the showLoginPage config to be overridden per request via an X-Show-Login-Page HTTP header
• This way, E2E tests can send X-Show-Login-Page: false in the request, while manual testing can still be done in the browser, all against the same running instance of Mockpass

[LoneRifle]

lgtm otherwise!

[LoneRifle]

How about something more concise like..

Suggested change

	if (req.header('X-Show-Login-Page')) {
	return req.header('X-Show-Login-Page') === 'true'
	}
	return process.env.SHOW_LOGIN_PAGE === 'true'
	return process.env.SHOW_LOGIN_PAGE === 'true' || req.header('X-Show-Login-Page') === 'true'

[whipermr5]

It was actually to support a possible use case where the SHOW_LOGIN_PAGE env is true but the user wants to override it to false in a request via a header X-Show-Login-Page: false.

This was my main use case:

This way, E2E tests can send X-Show-Login-Page: false in the request, while manual testing can still be done in the browser, all against the same running instance of Mockpass

[whipermr5]

I considered this form, but thought it might be a bit hard to understand:

showLoginPage: (req) =>
    req.header('X-Show-Login-Page')
      ? req.header('X-Show-Login-Page') === 'true'
      : process.env.SHOW_LOGIN_PAGE === 'true',

[whipermr5]

Oh we could also do something like this:

showLoginPage: (req) =>
    (req.header('X-Show-Login-Page') || process.env.SHOW_LOGIN_PAGE) === 'true',