Add delete flag in configuration

[rpullinger]

- Summary
Fixes #593. Adds a delete flag to collections in config.yml. Defaults to false. This is mostly for use with files to restrict users from deleting settings files etc that are available via the CMS but can also be used to restrict the ability to delete within any collection.

I'm currently not 100% sure about the how it should handle delete not being set for a collection. Ideally I'd have it like I've implemented where if delete is not set then it counts as false. This is how the create flag works. My only worry is that when this feature is implemented and users upgrade they will suddenly find that unless they add the flag that they can't delete documents they could previously?

EDIT — This now defaults to true

- Test plan
Manually tested using example config using yarn run start.

• Testing delete: true – Visit http://localhost:8080/#/collections/posts/entries/2017-10-18-post-number-9 and ensure DELETE button is visible and deletes the entry.
• Testing delete: false –  Visit http://localhost:8080/#/collections/settings/entries/general and ensure DELETE button is hidden.
• Testing delete not set –  Visit http://localhost:8080/#/collections/kitchenSink/entries/a-big-entry-with-all-the-things and ensure DELETE button is hidden.

- Description for the changelog
Adds a delete flag to collections in config.yml to restrict users from deleting important documents via the CMS.

[erquhart]

@rpullinger awesome, looks good! One change, though: this needs to default to true, at least for now. We can consider changing it for the 1.0 release, but we want to avoid breaks unless absolutely necessary.

I would also say just add a single example of this option's usage in the example project, trying to reduce the signal-to-noise ratio in there.

Let me know if you have any other thoughts on any of this. Thanks again!

[Benaiah]

@rpullinger I've pushed the changes @erquhart requested into the delete-flag-in-configuration branch in this repo (netlify/netlify-cms) - if it works for you I can push them to this branch as well (or you can pull them from that branch) and we can merge (this LGTM once @erquhart's changes are addressed).

[rpullinger]

I've added @Benaiah's changes to the branch (cheers @Benaiah!) so that the default is now true. The config.yml now only has one example of the delete flag as well.

@erquhart – let me know if there is anything else?

[tech4him1]

LGTM

[Benaiah]

LGTM