Forums: Detect non-image URLs in <img> tags

[SergeyBiryukov]

With the upgrade to bbPress 2.x, it's now possible to insert images directly into support forum posts.

However, most new forum members incorrectly use <img> tags for web pages instead of images, which results in a broken image in their post. A recent example:

<img src="https://gyazo.com/c57448863a8cd682d3bc6ba718506547" alt="Shipping Options" />

The URL here is a web page with an image, but not the image itself.

This should be more user-friendly. We could probably detect that the URL provided for src is not an image and automatically convert the code to a regular link in that case:

<a href="https://gyazo.com/c57448863a8cd682d3bc6ba718506547">Shipping Options</a>

[SergeyBiryukov]

The img button was removed in [5737], so this should no longer be an issue.

#2097 would be the preferred way of inserting images into posts.