Add support for properly functioning navbar submenus

[rich-iannone]

This fix is for replacing the class of a submenu item (3rd level down) such that it gets the dropdown-submenu class.

Fixes #721

[rich-iannone]

For the _site.yml of:

name: "site"
navbar:
  title: "My Website"
  left:
    - text: "Home"
      href: index.html
    - text: "About"
      href: about.html
      menu:
        - text: "Sub_1"
          href: sub1.html
        - text: "Sub_2"
          menu:
            - text: "Sub_2_below"
              href: sub2.html

this change targets the second to last item (with text Sub_2) so that the sub2.html is available and clickable. Here is a screenshot:

This uses a CSS path that targets any item with this type of relationship.

This script generates files used to generate the site (and does the site render also):

index <- '---
title: "My Website"
output: html_document
---

Hello, Website!'

about <- '---
title: "About This Website"
output: html_document
---

More about this website.'

sub_part_1 <- '---
title: "About This Website"
output: html_document
---

sub_1: This is right below the About page.'

sub_part_2 <- '---
title: "About This Website"
output: html_document
---

sub_2: This is 2 levels below the About page.'

write(index, file = "index.Rmd")
write(about, file = "about.Rmd")
write(sub_part_1, file = "sub1.Rmd")
write(sub_part_2, file = "sub2.Rmd")

rmarkdown::render("index.Rmd")
rmarkdown::render("about.Rmd")
rmarkdown::render("sub1.Rmd")
rmarkdown::render("sub2.Rmd")

render_site()

[rich-iannone]

The submenu now functions properly and jQuery was not used in this final solution. Here is a screenshot of the menu/submenu:

Some CSS rules had to be added to 'restore' support for the dropdown-submenu CSS class (removed in Bootstrap 3.0 RC, see https://stackoverflow.com/questions/18023493/bootstrap-dropdown-sub-menu-missing for additional context).

The R code changes occurred entirely in the navbar_links_tags() function. By checking the depth, we can distinguish between a top-level menu element or a menu element below top level. Then, we can apply the dropdown or dropdown-submenu class to the li element.

[rich-iannone]

Forgot to mention it earlier... thanks @jcheng5 for the assistance on this!

[yihui]

Looks good to me! Thanks!

[yihui]

We probably don't need the -webkit and -moz prefixes: https://caniuse.com/#feat=border-radius

[rich-iannone]

Nice, I'll take those lines out.

[yihui]

Could you indent by 2 spaces and add spaces after colons (to be consistent with previous CSS code)?

[rich-iannone]

Sorry for missing this. Will do!

[yihui]

We also need a NEWS entry for this feature (or perhaps bug fix).