Domain Management: add unlimited email forwarding

[jeremeylduvall]

The Business bundle allows for unlimited email forwards. Currently though, Business users are still limited to 5 email forwards in Calypso. This PR does a few different things:

1. Create index.js in client/lib/domains/email-forwarding/.
2. Add a function emailForwardingPlanLimit that accepts product_id as a param and returns the allowed amount of email forwards.
3. In client/my-sites/upgrades/domain-management/email-forwarding/email-forwarding-add-new.jsx, this current email forwarding list length is compared against the return value from emailForwardingPlanLimit so the limit is set to 5 for Premium users and 100 for Business users (Easy to bump up, but 100 seemed pretty high).
4. The limit at the bottom of the email forwarding page ("You are using _ out of _ email forwards.") uses the return value from emailForwardingPlanLimit to set the correct number for the "out of" value. In order for this to work, I added selectedSite as a prop so I could reference it later on.

To test:

1. Setup a site with a Business bundle
2. Start adding email forwards. You should see a limit of 100.
3. Remove the Business bundle. The limit will drop back to 5. If you're at or over 5, the Add New button is disabled.

Here's how this currently looks on a site with a Business bundle active and 6 email forwards setup:

With 6 email forwards setup, I removed the Business bundle and the limit is set back to 5:

Fix for #2387. h/t to @bikedorkjon for writing most of the code in the original bug report. @gziolo is the kind of setup you were thinking of?

[drewblaisdell]

In general, we try to avoid including a product ID or slug at a random place in the code like this, in favor of functions that do the number/string comparison for us. This makes it easier for us to change the product ID or slug, as we only need to modify the function that does the actual comparison.

Currently, we use isBusiness for this, which lives in lib/products-values. If you change this function so that it accepts a product (which, in this case, is the plan), you could use that function instead of this comparison (i.e. isBusiness( plan )).

[jeremeylduvall]

Appreciate the feedback @drewblaisdell! Updated the function emailForwardingPlanLimit to accept plan instead of product_id. Used isBusiness() in the logic now instead of comparing the product_id. Still works as originally anticipated.

[gziolo]

You can simplify that to:

return ( isBusiness( plan ) ? 100 : 5 );

[gziolo]

Code looks good 👍 This is exactly what we need :D
I haven't tested it.
It just needs sign off from @bikedorkjon and can be 🚢

[jeremeylduvall]

Thanks @gziolo! Changed the emailForwardingPlanLimit function to just be return ( isBusiness( plan ) ? 100 : 5 );. Much simpler. Still works as originally anticipated. Appreciate it!

[bikedorkjon]

I was able to add 7 email forwards. Tested and LGTM! Nice work, @jeremeylduvall :)