Replace Language image references with CSS

[Vailorium]

The language icons in various places currently use long conditional statements to display the different images, it would be good to replace these image references with CSS classes instead (where the CSS class is the language + the variant of the language, if any).

---

Eventually I would like to see this change (language variant icons) to instead be a single line that uses code along the following lines:

if (variant != null && !variant.isEmpty()) {
    // set the img class property to be 'language-variant' (e.g. 'java-android')
} else {
    // set the img class property to be 'language' (e.g. 'java')
}

Then move all of the image file references out to CSS, e.g.

img.java {

}
img.java-android {

}

Originally posted by @JonathanGiles in #3797 (comment)