Extract email features into separate file and clean up Titan feature flag code

[daledupreez]

Changes proposed in this Pull Request

• This PR extracts the feature lists for Titan, Google, and Email Forwarding into a separate to facilitate re-use for upcoming updates to the email comparison page.

Testing instructions

• Using a local build or a live branch, try to add email to a domain you already own. Verify that the features for all three options appear.

[matticbot]

Test live: https://calypso.live/?branch=update/refactor-email-comparison-features

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~194 bytes removed 📉 [gzipped])

name   parsed_size           gzip_size
email       -533 B  (-0.1%)     -194 B  (-0.2%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[hambai]

Tested and those changes work as expected!