Ampersands in URLs are no longer converted to entities

[SergeyBiryukov]

Reported by @hoveroverus on support forums.

1. Go to Add New Post screen.
2. Switch from TinyMCE to the Text editor.
3. Paste this to the textarea: & <a href="http://example.com/?a=1&s=2&hl=3">test</a>.
4. Click the Preview button.

In 4.3.1, all ampersands are converted to entities:

&#038; <a href="http://example.com/?a=1&#038;s=2&#038;hl=3">test</a>

In 4.4, the ampersands in the URL are not converted, which leads to errors reported by W3C HTML validator:

&#038; <a href="http://example.com/?a=1&s=2&hl=3">test</a>

[ocean90]

See ticket:34698:18.

[pento]

In 36036:

Texturize: Transform & into & in tag attributes.

[35709] was overly broad, and stopped transforming & characters within tag attributes. So that sites aren't generating invalid HTML, we need to restore this functionality, while continuing to not transform & within blocked tags.

Fixes #35008 for trunk.

[pento]

In 36037:

Texturize: Transform & into & in tag attributes.

[35709] was overly broad, and stopped transforming & characters within tag attributes. So that sites aren't generating invalid HTML, we need to restore this functionality, while continuing to not transform & within blocked tags.

Merge of [36036] to the 4.4 branch.

Fixes #35008.