Add Apple Pay Setup for Donorbox

[raclim]

Changes:

• adds required setup in order to add Apple Pay integration to Donorbox in .well-known/apple-developer-merchantid-domain-association
• configure server.js to explicitly set the file's Content-Type as text/plain vs. the default application/octet-stream

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[release-com]

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