sortableFields have default sorting option

[KancerEzeroglu]

closes #3715

Summary
We couldn't set the default sortable field from the config file.
With my changes, it can happen.
You can specify the sortable fields like below:

sortable_fields:
   - field: title
      direction: 'Descending'
   - field: date

Test plan
I haven't added the tests in the code (I've removed those that belong to the sortable fields test).
I would like to be sure my direction is right.

A picture of a cute animal (not mandatory but encouraged)

[erezrokah]

Hi @KancerEzeroglu and thank you for this.
A few comments:

1. We would like to keep the config backwards compatible. With this PR anyone with existing sortable_fields will have to update their config. We can add migration code here:
   https://github.com/netlify/netlify-cms/blob/567438fdd1aa775fd09a6a5de13e79c9d783e0d6/packages/netlify-cms-core/src/actions/config.js#L283

2. The current suggested API allows setting multiple fields as the default which is not supported by the UI. We can use something like:

sortable_fields: { names: ['title','date'], default: name, direction: descending }

3. I think it would be better to initialize the default sort in the reducer to avoid the UI refreshing after initial load.

WDYT?

[KancerEzeroglu]

Sorry for the late reply @erezrokah
I will work on what you have written and get back to you.
Thank you for your review.

[KancerEzeroglu]

The current suggested API allows setting multiple fields as the default which is not supported by the UI. We can use something like:
sortable_fields: { names: ['title','date'], default: name, direction: descending }

In the future, we can enable users to do multiple sorts.
So I think the configuration should be based on this.
What do you think about that @erezrokah?

[erezrokah]

In the future, we can enable users to do multiple sorts.
So I think the configuration should be based on this.
What do you think about that @erezrokah?

Hi @KancerEzeroglu and sorry again for the late reply.

How about enforcing using our schema validation that only a single field has a direction?

[erezrokah]

Closing this as stale. Let me know if you'd like me to re-open it

[jlev]

It would be really nice to have default sort ordering for date fields. With a lot of posts, having the default be date ascending is not a great user experience...

Is the direction schema validation a blocker to getting this merged? If so, I can take a look at it if you point me in the right direction.

[erezrokah]

Is the direction schema validation a blocker to getting this merged? If so, I can take a look at it if you point me in the right direction.

Hi @jlev the blockers for the PR are:

1. It breaks existing configurations.
2. It allows you to configure something that doesn't work (can be solved by schema validation)

Please note that the CMS remembers the user's preference for the sort if that helps and having a default sort field can impact performance quite a lot as it requires the CMS to fetch all posts on initial load.