Simplify use of fontawesome icon for navbar - no prefix required

[cderv]

Following the different PR we have regarding fontawesome

• rmarkdown did upgrade to FA5 (rstudio/rmarkdown#1340)
• but the new prefixes where not supported in navbar, so I extended support in rstudio/rmarkdown#1967 making the new prefix fas the default
• But this broke some stuff (the issue you read) that where working without any prefix because it was added but rmarkdown. Hence the last PR (rstudio/rmarkdown#1994)

After @gadenbuie challenge a bit the current fix, I looked into how shiny handles it : Basically the maintain a list of brands icon and add the fab prefix if needed. This helps for using V4 and V5 with simply the name of the icon.

Also it uses an update script that would be useful to easily update the dependency here too. I believe keeping in sync the version in both shiny and rmarkdown is better.

So we should do something inspired by the logic in shiny, and maybe this would solve also what rstudio/rmarkdown#1994 is trying to solve.

[cderv]

Related also to the update of FA: #1661

[cderv]

We'll use the new on CRAN fontawesome package now.

[rich-iannone]

Note: this may work now with the new fontawesome dependency, need to test to confirm.