Add Nunjucks filters for formatting dates

[gonogo]

This Nunjucks filter implementing moment.js to allow formatting of dates

eg. {{ tx.lastUpdated | date("DD-MMM-YY") }}

results in:

13-JUN-19

[NickColley]

Hey @gonogo,

These look great!

Would you be willing to reduce the scope of your pull request to include only the date filter which has been requested a few times in the past: #170

If we get more evidence that a lot of people need the other filters we can consider them separately.

If you're OK with this then we'll also need tests and documentation to explain this functionality.

Let me know if you need any help with any of this, thanks again.

Nick

[gonogo]

@NickColley I've now removed the other two filters - will submit as separate pull requests.

[gonogo]

This should be complete now, pending CI checks.

[NickColley]

Thanks a lot!

Would you be willing to write some tests and documentation for this feature?

[gonogo]

Closing this, realistically there's no time left for me to do any more work on this as my contract finishes today. Might reopen this and get some work done on this next time I do some work on a GOV.UK project.