normalize pre tags after redactor conversion

[i-just]

Description

Redactor and CKEditor use slightly different markup for code block content. Redactor wraps such content in a <pre> tag, while CKEditor requires <pre><code class="language-{langId}"> by default. This means that all <pre> tags that are not followed by a <code> tag need to be normalised to contain them.

To prevent this normalisation from interfering with custom configuration of the CKEditor code block feature, the code will only convert <pre> to <pre><code class="language-plaintext"> if the <pre> tag is not immediately followed by the <code> tag.

Original Redactor field:

Redactor field converted to a CKEditor field before this PR:

Redactor field converted to a CKEditor field with this PR:

Related issues

#258