Fix CSSJanus flipping in LESS mixins and remove broken custom LESS functions

Custom LESS functions are problematic for us for a number of reasons,
as outlined by Timo on bug 67368. We should get rid of them.

The only use case was implementing CSSMin data: URI embedding in LESS,
which used to be impossible due to lessc not preserving comments (bug
54673). However, thanks to new syntax added in f3779e06 we can insert
the annotations in such a way that the compiler won't mess with them.
The same technique is used in OOjs UI since 584ed144.

The LESS-function-based embedding implementation also meant that we
were unable to flip images for RTL (bug 66091 and friends: bug 66773,
bug 68326). The annotation one doesn't have this limitation.

Bug: 67368
Bug: 66091
Bug: 66773
Bug: 68326
Change-Id: I3062346ed63272a1c22b5df27b4cc1de2a699d9a