This PR introduces the ability to just override a LESS file's contents through an extender.
This is mainly useful for theme development, as there are times in extensively customized themes where overriding the actual file makes a huge difference vs overriding CSS styles which can turn into a maintenance hell real fast.
Overriding styles is more tedious than overriding files. When you're designing an element, you would normally rather start from a blank canvas, than a styled element. With an already styled element you have to first override and undo the styles you do not wish to have, only then can you start shaping it, but even then you'd always end up constantly undoing default styles. This mostly applies for more advanced themes. (example: 851c55516d/less/forum/DiscussionList.less)
* Write source map without creating temp file
Less I/O, and one less place where we access the global path helpers.
* Drop useless app_path() helper
This was probably taken straight from Laravel. There is no equivalent
concept in Flarum, so this should be safe to remove.
* Deprecate global path helpers
Developers using these helpers can inject the `Paths` class instead.
* Stop storing paths as strings in container
* Avoid using path helpers from Application class
* Deprecate path helpers from Application class
* Avoid using public_path() in prerequisite check
a) The comparison was already outdated, as a different path was passed.
b) We're trying to get rid of these global helpers.
