After Vagrant setup, recommend testing in browser

The curl command seems to throw some volunteers for a loop. The output
of curl does not make the next step obvious and even with instruction,
people sometimes run the command inside the VM.
Remove note about starting Django, node manually

We recommend the use of foreman start earlier in the documentation, so
this should be unnecessary.
Create new "Developing with Vagrant" doc section
Add docs about creating or importing wiki pages
Merge pull request #2221 from openjck/vagrant-doc-improvements

Improve Vagrant installation instructions