Added council suggestions for comparison tool

[lucascumsille]

Fixes: #686

I left some comments regarding things that are pending, but front-end related I think it should be ready.

• We need to pull a list of 5 councils based on overall similarity like we do in CAPE. I picked 5 randomly, it seemed like a sweet spot between giving the user enough options, but not overwhelm them with too many choices.
• Replace the placeholder councils with a loop based on overall similarity.
• We need to add commas to the list of councils, but I can do that using CSS, however I thought we could keep doing it with Django like we have done in other part of the website. Let me know which approach you prefer.

Preview here:

[lucascumsille]

@zarino and @struan I think I'm done with this PR, but we need the back-end logic to be implemented. I left some comments at the beginning.

[zarino]

@lucascumsille I’ve wired this up, so that a maximum of 5 overall related councils are passed to the template, removing any that are already selected for comparison. I simplified some of the wording, and used flex layout for the links, rather than concatenating them into a sentence with commas.

[zarino]

Marking as ready for review by @struan, so he can check I my Python isn’t too horrendous, and work out whether we should fix the linting errors or not.

[struan]

this looks good.

I've pushed some commits to update the black version and fix the resulting linting issues. We were using an old version so I've updated it to the latest version.

[struan]

why does this only apply if it's composite?

[zarino]

We only want the 5 "overall" similar councils, not the other types of similarity.

[zarino]

(I just copied this block, wholesale, from the caps app)