Add link to manage menus to customizer

[celloexpressions]

"Your theme supports 2 menus. Select which menu appears in each location. You can edit your menu content on the Menus screen in the Appearance section."

We should provide a direct link, in addition to explaining where to edit your menus. The admin menu isn't even available in the customizer, so I'm not sure how useful that explanation is anyway.

[celloexpressions]

Potential link placement

[celloexpressions]

Not sure if I handled the i18n correctly, or if we need to use the full URL for any reason.

[nacin]

Yep, we need to use the full URL. So it should be %s and then substituted in.

The problem with this, though, is we're linking to a dashboard page from the customizer, which would unexpectedly close the customizer. The link is actually *less* useful than not having it at all.

[celloexpressions]

Replying to nacin:

The problem with this, though, is we're linking to a dashboard page from the customizer, which would unexpectedly close the customizer. The link is actually *less* useful than not having it at all.

Twenty Fourteen adds a couple of links to customizer section descriptions, so this would be behavior that already exists in new installs by default (guessing you aren't aware of that?). I think leaving the customizer like this is okay, although it can cause changes to be lost. Alternative options would be to open in a new tab (bad for accessibility) or to wait until autosaving functionality makes it into the customizer. Or, we could add a warning onunload if there are unsaved changes (I believe there's history with that approach, but can't remember details).

Related, considering the concern about linking: #26890.

[celloexpressions]

​http://make.wordpress.org/core/2014/06/28/gsoc-menu-customizer-update/

[SergeyBiryukov]

Replying to celloexpressions:

Or, we could add a warning onunload if there are unsaved changes (I believe there's history with that approach, but can't remember details).

Related: #25439