Fix footnotes in legislative lists

[deborahchua]

What

This fixes an issue with footnotes within legislative lists in Whitehall.
Trello card: https://trello.com/c/yfeqz1no/2561-footnotes-dont-work-within-legislativelist-on-manuals-publisher-5

Why

This has been a longstanding issue and is heavily used in legislation papers.
It means that publishers cannot use footnotes within certain kinds of content, and so cannot always follow the design guidelines. Having markdown that's reliable and effective is helpful for selling our accessibility approach (avoiding PDFs) to policy colleagues.

Visual Changes

Before (footnotes in a legislative list not converted into html):

After (footnotes in a legislative list now showing):

Before (footnote definitions missing):

After (footnote definitions now showing):

[ChrisBAshton]

Well done for working through this fiddly issue! I've got some code style comments below, but otherwise this looks basically ready to go. However, there is something I'd like to check first. From the first commit:

We currently don't have the ability to render footnotes in Legislative
lists. This workaround substitutes the footnote anchor links (e.g. [^1]
for it's equivalent html.

Do we know why we don't have the ability to render footnotes in Legistlative lists? Is there just a method call we need to invoke somewhere? It feels awfully cumbersome to have to recreate our own footnotes/acronyms markdown parsing logic if it already exists and works elsewhere. Apologies if you've already looked into this, I'm viewing this with fresh eyes and haven't looked too closely at the coal face.

My only other question was whether we're duplicating any of this code - it particularly feels as though the "go to where this is referenced" bit should be encapsulated in one place rather than repeated here.

[deborahchua]

Thanks @ChrisBAshton! Addressing your comments below:

We currently don't have the ability to render footnotes in Legislative
lists. This workaround substitutes the footnote anchor links (e.g. [^1]
for it's equivalent html.

Do we know why we don't have the ability to render footnotes in Legistlative lists? Is there just a method call we need to invoke somewhere? It feels awfully cumbersome to have to recreate our own footnotes/acronyms markdown parsing logic if it already exists and works elsewhere. Apologies if you've already looked into this, I'm viewing this with fresh eyes and haven't looked too closely at the coal face.

So this was the part I spent a great deal of time trying to figure out a better workaround. Leglislative lists was a new markdown tag introduced a while back so that editors can have custom numbering for ordered lists and different indentation levels (see this commit which explains in detail) The approach used means a sub document is created and the footnotes only exists inside that document, with some hacks to the Kramdown to make it work.
Going by the previous comments and my understanding of the problem, this icky workaround seemed like the best option without having to create our own custom markdown parser for lists.

My only other question was whether we're duplicating any of this code - it particularly feels as though the "go to where this is referenced" bit should be encapsulated in one place rather than repeated here.

The html for footnotes doesn't exist at this stage if the document has footnotes in legislative lists, which is why it needs to be manually processed at an earlier point

[ChrisBAshton]

Thanks for the explanation, and for making the changes, @deborahchua! This looks good to go 👍 I've got a couple of nitpick comments below, but they're non-blocking.