Allow for namespacing of modal (and backdrop) window

Created by: JeanMertz

I made this change for my personal use a few days ago but figured it might be something others would want. Not sure if I implemented it correctly (but I tested it, and it works) as I am far from being a Javascript/jQuery expert.

This commit makes it possible to namespace your modal windows.

The use case for this is a page where you allow for clients to have custom html & css, but also have some static html/css of your own. Not namespacing your own css means that it could be overwritten by accident by the custom code.

I still wanted to use the Bootstrap elements so I simply added a class namespace inside the LESS files by wrapping all the css inside a new block:

.my_custom_namespace {
  ... original bootstrap css here ...
}

This is very easy to do yourself and it works great, except that I can't use the vanilla JS with this because it places elements directly in the body of the page instead of the namespaced element.

With this commit, if you want a different parent element than the document.body, you pass it the parent option.

$("#my_modal").modal
  show: true
  parent: ".my_custom_namespace"
  backdrop: "static"

Now the modal will be positioned inside the element with class: my_custom_namespace on hiding/showing.

If this is any good, and people are interested, perhaps it would be a good idea to port this functionality to the other JS files as well, but I haven't had a chance to check those out yet.