Allow embed <iframe> in job description content

[jom]

Fixes issue of embeds getting removed from job description content. Introduced by #1495 in 1.31.1 (July 2018).

Fixes #1584
Fixes #1718

Changes proposed in this Pull Request:

• Allows iframe in job description content.

Testing instructions:

• Add YouTube link in job description.
• Verify embed works correctly on frontend.

To Do

• Add tests

[tripflex]

While I understand the need for something like this --- doesn't allowing iframe in job content open up sites for potential exploitation by injecting malicious iframes?

[jom]

@tripflex We could add wp_kses_post before it passes through the_job_description (which adds the embeds; like the_content filters). I’d be fine with that and should still fix this issue.

Trying it out here: 49c4927

Seems to still work. It adds a bit of extra overhead, but probably worth it.

[alexsanford]

Seems to work well. One question about how the kses is happening, but otherwise I think this one looks good!