Site Settings: Add date and time format settings to the endpoint

[Copons]

Changes proposed in this Pull Request:

• Add date_format, time_format, and start_of_week settings to the site settings endpoint, in an effort to bring these settings to Jetpack sites too in Calypso.

Testing instructions:

• Run Calypso on this branch, open the site settings of a Jetpack site: the date and time format settings are populated.
• Change them and save them: after a reload, they are populated with the new value.

Please disregard if the values are a bit off: the conversion method between PHP and JS formats is still in progress.
Also disregard the alert on reload: it's a known issue, unrelated to this change.

[rcoll]

Howdy @Copons this looks good to go and the Jetpack side of things appears to work properly. I do see an issue in the Calypso branch (maybe you're already aware or it's intentional) - I'm unable to select a custom date or time format. When I click the radio for "Custom", the setting "bounces" back to where it was.

[Copons]

@rcoll ah yup, it's a known UX awfulness that will be changed shortly.
Thanks for the review!