[UX] Add a token-browser-like PHP date format helper dialog

This is a spin-off from #4536.

@BWPanda

There's a bit of scrolling needed to find the actual table of formats, and no way to link straight to it from what I can see. Since I doubt those formats would change much, I wonder if it'd be better for us to create our own page to link to...? Either in Backdrop itself (in a modal like the tokens one), or on b.org/api.b.org.

@jlfranklin

The Date/Time Format Help module will include the documentation directly on the "Add date format" page.