Make appending paths more DRY

[DrewAPicture]

In working on a plugin recently, I came across a couple of lines in core that effectively append a specified path to a URL in several functions, namely home_url(), content_url(), plugins_url() and so on.

By my count, these same two lines are used in at least 10 functions, so I propose we wrap them into a new append_path function, though the name could be reworked.

As a side note, I'm not a huge fan of assigning $url twice but don't feel like this falls under the pervue of set_url_scheme() at all. Open to suggestions, though.

Patch attached.

[wonderboymusic]

Yep - did that here: #22666