Add delay message

[NickGreen]

Adds delay message to the update message box for each eligible plugin on the plugins page.
Displays date in format as set in the site settings, in UTC.

[georgestephanis]

I'm not certain on the wording -- I think my brain would prefer the append as something like Version ##.## will not autoupdate until at least MM/DD/YY or the like -- I appreciate how we're using the site's options for date and time format -- but I'm largely going to defer to TAMs for how they feel the wording would work best for partners.

[NickGreen]

Updated the wording as per TAM suggestion.

@georgestephanis @ecairol Would either of you dudes be willing to take a look at the code to make sure it's right and safe, before we merge it and send it off to a bunch of sites? :)

[ecairol]

LGTM! Left a couple of comments.

[NickGreen]

Also added checks to not show the message if:

1. Autoupdates aren't available (i.e. wc.com isn't connected, so paid plugins can't update)
2. Plugin is deactivated (autoupdates don't happen when plugins aren't active)

Tested, merging.