Replace deprecated String.prototype.substr usage

[scrhartley]

Description

Inspired by the already merged in extensions PR #100.

On String, the substr method is deprecated and is not part of the main ECMAScript specification
(see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr).
Replace with the alternatives slice and substring, which are both already used in the codebase.

Testing

I made sure that giving bad values caused the existing test cases to fail.
I temporarily added additionally logging to verify that it was doing what I expected.
I checked that most of the changes are trivial so that this could be considered as a single change.

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded

[alexpetros]

Yup, seems like a good idea.

[1cg]