Fixes#4602 as per https://github.com/wagtail/wagtail/issues/4602#issuecomment-479539444 (option 2).
Previously, given HTML input such as:
<p>
<i>a bunch of text before <embed alt="somepic" embedtype="image" format="fullwidth" id="1"/> after</i>
</p>
the `<embed>` would start a new block, but the converter would keep hold of references to currently-open tags such as the `<i>`, so that when the corresponding `</i>` tag was encountered, it could match it up to the opening tag and fill in the 'length' field on the resulting InlineStyleRange object. However, since the span length is calculated based on the text content of the _current_ block (which is now "after"), it would obtain the wrong result - or, when there is no content between the embed and the closing tag (and thus no current block), would throw the exception `'NoneType' object has no attribute 'text'`.
In this new approach, when the embed is encountered, the current block is closed _along with all of its styles and entities_, causing the lengths of those spans to be filled in correctly. After inserting the embed, the current block is then set to a replica of the previous block with all those styles and entities reopened, so that when the closing tag is finally encountered, the span length is correctly set based on the new 'after' block.
This replaces several US English dialectical uses of _regular_ with
British English equivalents _normal_ or _standard_. This is the result
of a search of the docs for the string 'regular', rather than due to any
US English seen in the user interface.
This search also found one use of _regularly_ where _often_ was closer
to the intended meaning, a change which is less about dialect than about
word choice.
Some edit handlers, such as the 'unofficial' PerUserContentPanels recipe from #4749, vary their field list according to the current request/instance by hooking into bind_to. This was not being called on the comparison view, meaning that when these edit handlers are in use, the field list was never getting populated and so the view was wrongly reporting no changes.
Note that the bind_to method also allows binding a form, which we do still skip (since the comparison view doesn't construct one).
This means we're not artificially forcing four different entity types into the same code path, and makes it possible to define new entity types outside of this module.
Also relax the eslint no-unused-vars to allow unused function parameters - having multiple classes following the same interface is a legitimate use of this.
Since Wagtail 2.7, this is no longer true by default when using remote storage - only when `WAGTAILDOCS_SERVE_METHOD` is explicitly set to `serve_view`.
Improve the generation of `<title>` tags as follows:
* use `page` in preference to `self` (self has been semi-deprecated ever since we added jinja2 support)
* Retrieve current site with `{% wagtail_site %}` rather than page.get_site so that it works on non-pages such as 404s
* Fill in the 'title' block on 404.html
This prevents duplicated headings in places where the second-level index page has intro blurbs for each subsection (and also means we're not listing out long multi-section pages in full in the index, but given how big these indexes are that's probably not a bad thing).
Previously, this would fail, as `user_can_copy_obj` doesn't handle root
pages. Root pages also are special in a number of ways, and handling
copying is tricky and non-obvious.
If someone needs to show root pages (for some reason), they'll have to
special case them themselves.
Matt Westcott