WIP: Generate tokens with variable expiry date

[rhwilr]

Hi @thetutlage

I added the expires_at field to tokens as we discussed in #24. So far it works, but I have two questions I'd like to get your input on:

1:
In order to allow you to set a different duration for each token, I had two options: Add a parameter to all methods (generateToken, register, updateEmail, updateProfile) that generate a token, or add a chainable method. I went with the latter since it would have made it a bit awkward to pass the parameter down. And I especially didn't like the signature for register:

async register (payload, callback, validFor = this.config.validFor) {

So currently I use the isValidFor method to set the parameter and return this to allow chaining. But this means I have to reset the value after generating a token.

I'm not happy with either of those options. So I want to ask you what you prefer or if you have a better idea?

2:
What should happen with existing tokens when users upgrade?
Currently, I treat tokens without an expiry date as invalid. To make the migration easy we could add a fallback: If expires_at is null, we fall back to the previous behavior and check the updated_at field. However, this fallback would only ever be used for 24 hours after the upgrade.

I would prefer the fallback option. So I tried to construct a query, but it turns out to be quite difficult since the query parameter is sometimes a user.tokens() relationship which would lead to a wrong or clause.

This was my attempt:

  _addTokenConstraints (query, type) {
    const dateFormat = this.config.dateFormat

    query
      .where('type', type)
      .where('is_revoked', false)
      .where('expires_at', '>=', moment().format(dateFormat))
      .orWhere(function () {
        this.whereNull('expires_at')
          .where('type', type)
          .where('is_revoked', false)
          .where('updated_at', '>=', moment().subtract(24, 'hours').format(dateFormat))
      })
  }

Looking forward to your feedback.

[coveralls]

Pull Request Test Coverage Report for Build 43

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 93.846%

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Totals
Change from base Build 42:	0.3%
Covered Lines:	144
Relevant Lines:	149

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