Netlify-cms: Add `label_plural` field to collection config

[verythorough]

- Do you want to request a feature or report a bug?
feature

- What is the current behavior?
The label field for a given collection is displayed in all places in the UI where the collection is referenced. At the moment, this is not pluralized in any way, so a collection with label "Post" will display as simply, "Post" in the sidebar. If you change the label to "Posts," it looks good in the sidebar, but "New Posts" doesn't make sense.

We used to have programmatic pluralization, but it sometimes produced undesirable results, and was removed.

- What is the desired behavior?
It would be nice to have an optional label_plural field (or something similar), allowing implementers to specify a label version to use in cases where the UI calls for a plural. With this in place, we can bring back programmatic pluralization and use it as a default when the manual plural isn't specified.

[erquhart]

Along with this I'd like to reintroduce automatic pluralization as a fallback, it'll work for a lot of use cases.

[verythorough]

(which I did actually mention) ;)

[erquhart]

Yikes you totally did :woman_facepalming:

[DirtyF]

• [ ] use singular instead of plural for Collections on buttons (posts)

e.g. in those context, article should be singular

[erquhart]

Right now there is no pluralization happening in the cms, it just uses the configured collection name.

[peduarte]

Hello 👋 I'm happy to pick this issue up and raise a PR for it.

[erquhart]

Thanks @peduarte!