Add validation phone number for subscription

[u5surf]

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Fixes #5

Summary

I added validation phone number when we subscription the service.

Test Plan (required)

build lamdas/subscrition/main.go and lamdas/subscrition/main.go and then, to indicate validation, we'll be executing these to subscribe valid or invalid number case.

Final Checklist

• [-] For CoronaTracker, did you bump the version in package.json?
  • Update the second decimal for a major change
  • Update the third decimal for a minor change
  • Numbers can go past 9, e.g. 1.0.9 => 1.0.10
  • For more info, read about Semantic Versioning
• [-] Did you add any new tests as necessary?
• [x] Is your PR rebased off the most current master?
• [x] Have you squashed all commits? (can be done at merge)
• [-] Did you use yarn not npm? (important!)
• [-] Did you use Material-UI wherever possible?
• [-] Did you run yarn lint on the code?
• [-] Did you run all of your most recent changes locally to make sure everything is working?

[SomeMoosery]

So right now we're doing this check in the frontend as far as I know - do we think it could be worthwhile to add in the backend as well as a failsafe? Or does that seem redundant?

[acthelemann]

I like the idea of it in both places, but now I'm thinking it might be inconvenient if we want to change the regex/validation since we'll have to do it in both places.

[SomeMoosery]

So do you think we should close this for now and reopen if the need arises?