Style frequently used comments for bootstrap 4.

Also added support for the SCSS property to be a Closure
and return content rather than simply being the name of a file.

Changes include:
* question: Add bootstrap classes to buttons and selects so they look consistent.

* mod_quiz: Add bootstrap styles to report form elements

* question: Add bootstrap classes to question form elements

* And remove the success/fail styles from boost because they conflict badly
with the quiz plugin styles (making unreadable text).

* quiz: nav block buttons had black background

* quiz: Add bootstrap styles to quiz navigation buttons

* qtype_ddmarker: Remove bullets from list

Edit form coordinates preview had list bullets for each coordinate.

* boost: Prevent menu scrolling on quiz edit

* mod_quiz: Add bootstrap styles to edit quiz fields

Pagination and max grades forms.

* quiz: Undo some custom styles on review options

These checkboxes were laid out in a fake grid with no responsiveness
that conflicts with the bootstrap form-inline templates. Reset the
custom style to bootstrap default.

* quiz: Fix unit tests for boost changes

* qtype_ddwtos: Accurate positioning of drop targets

* questions: Make the review page responsive

* question: bootstrap 4 compatibility

.alert in bs4 contains no colours by default, you need to mix it with
one of the context classes to get some change in colours, borders etc.

Because Bootstrap Javascript does is not expecting to be flipped, it breaks the popover on RTL languages.
We already swap the directions on the help icon template so popover scss rules can be ignored.

1) Add example context where missing
2) Don't use an uncessary function in JS

This was present in bootstrapbase and is I think an important visual
distinction.

From bs4-compat.less, replace with file level disable instead.

month is displayed in table caption which is by default in the bottom in BS4

Added a link to create a new message from the message
popup menu.

The markup in the filemanager_fileselect template was being escaped,
causing a help icon not to display.

Additionally, the popover for the help icon was not display since
"loader.js" was not properly delegating the popover calls.

Don't put div in a span

The items in the flat navigation were double escaped. Also added
context to template.

Remove existing overrides forcing the markup off

No idea what happened to my testing of this change, but it was wrong.

Bootstrap makes select form elements not using custom-select broken when
they have a size. The contents overflow the field.

Use a bootstrap popover for the assign roles form.

This just injects bootstrap classes into the old yui2 thing. It needs
rewriting.

The main change is that in boost we get rid of the random centered text.
To compensate we use "container" which is fixed with with auto margins -
but the text is still left aligned.

Plus remove some pointless code.

Some pages (fresh install) don't have the navbar yet, so check for null.

I stayed away from big markup changes. This is a table layout so needs rewriting which is a big job.

I removed most of the layout styles and started again.