Add support for Django 5.1 by removing deprecated features

[jamesgilmorelyst]

There is only one Django feature used here that is removed in >=5.0, which is the length_is filter query:

RemovedInDjango51Warning: The length_is template filter is deprecated in favour of the length template filter and the == operator within an {% if %} tag.

It is a trivial change to make, and the length feature if backwards compatible to the earliest support version this repo supports of 2.2.

Preview of fieldsets, which has been modified in this PR:

As this is my first PR, I've also added in some QoL changes for new devs working in this repo. Hopefully that's okay 🙂

[PacificGilly]

Hi @farridav, sorry to be a pain, but I've migrated my GH account from my work to personal account so I can continue to make contributions for as long as possible. Would it be okay to help contribute/maintain from my current account instead, please?

[farridav]

Sure, makes sense

[afiestas]

Hey, just a headsup django just release the stable 5.1 so would be lovely if tihs PR could be merged soonish and a new version release.

Thanks for all the awesome work, much much appreciated 😊

[collinsinclair]

would love to see this merged in - this is now a blocking issue on a fresh Django project