Don't strip class names and ids from article content

[leibovic]

Right now we inconsistently maintain class names on readability content. It looks like the original intention was to actually strip all these class names from content, but we've found that preserving them can be useful for applying styles to common class names (e.g. .visibility-hidden should be hidden).

Now that we landed https://bugzilla.mozilla.org/show_bug.cgi?id=1154028, these class names shouldn't interfere with the reader view controls, so we could try to preserve even more class names for more special CSS rules.