Add Cloud Functions

[steffnay]

Fixes #23

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #24 into master will decrease coverage by 0.31%.
The diff coverage is 98.5%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
- Coverage   98.98%   98.67%   -0.32%     
==========================================
  Files           1        4       +3     
  Lines          99      227     +128     
==========================================
+ Hits           98      224     +126     
- Misses          1        3       +2

Impacted Files	Coverage Δ
system-test/functions_system_test.js	100% <100%> (ø)
src/util.js	99% <100%> (+0.01%)	⬆️
system-test/issuePayload.js	100% <100%> (ø)
functions/index.js	96.72% <96.72%> (ø)

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[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

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Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this state. It's up to you to confirm consent of all the commit author(s), set the cla label to yes (if enabled on your project), and then merge this pull request when appropriate.

[ace-n]

LGTM, other than a few nits.

[ace-n]

Missed a spot? 😛