Enable user-configured commit message templates

[delucis]

Summary

Closes #557

Exposes a backend.commit_messages option, which allows users to configure the commit messages generated by Netlify CMS using string templates. Default behaviour remains unchanged and is equivalent to the following in config.yml:

backend:
    commit_messages:
        create: Create {{collection}} “{{slug}}”
        update: Update {{collection}} “{{slug}}”
        delete: Delete {{collection}} “{{slug}}”
        uploadMedia: Upload “{{path}}”
        deleteMedia: Delete “{{path}}”

Why?

Users may want Netlify CMS commits to be formatted in a specific way. For example, they might want to adhere to a commit message convention they are already using. More discussion can be found in #557.

To-do

• Write documentation. Should this be under Beta Features?

Considerations/Notes

• Previously, persistEntry and deleteEntry were passed a config object (in actions/entries), while persistMedia and deleteMedia were not (in actions/mediaLibrary). To allow configuration of commit messages for media uploading and deletion, I changed
  persistMedia and deleteMedia to also receive a config object. For consistency with the entry methods, this is passed as their first argument. As far as I can tell, this only impacted actions/mediaLibrary, which I updated accordingly (3e360d1). Discussed below

• The variables available are pretty minimal and inconsistent based on what is available in each commit method at the moment. For entry commits, slug, path, and collection are available, but for media commits only the path variable is available. It would be nice if this were all standardised somehow in the future with some kind of commit object holding more information.

• Netlify CMS also creates commit messages when merging and force merging pull requests in its GitHub API implementation. In theory, these messages should also be configurable, but perhaps that can wait?

• Should the config option be elsewhere? @Benaiah mentioned a future backend.settings namespace for backend-specific options.

• I duplicated the string template logic from the slugFormatter. Given that this curly braces syntax is now in use in at least two places, perhaps the template-processing logic should be abstracted?

Test plan

• npm run test passed
• Tested in a test site running on Netlify (using the one-click-hugo-cms template)

Description for the changelog

Enable user-configured commit message templates

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

A messenger pigeon 🐦

[verythorough]

Deploy preview for netlify-cms-www ready!

Built with commit f45cec5

https://deploy-preview-1359--netlify-cms-www.netlify.com

[verythorough]

Deploy preview for cms-demo ready!

Built with commit f45cec5

https://deploy-preview-1359--cms-demo.netlify.com

[erquhart]

Great PR, thanks for putting this together! Just one small change request. Agreed that documenting this under beta features makes sense for now.

[erquhart]

Switch the argument order for persistMedia and deleteMedia so the config is the last argument.

[delucis]

Ok, will do. One note: I decided to pass config as the first argument because that is what persistEntry and deleteEntry currently do. Not sure if inconsistency between entry and media actions is a concern.

[erquhart]

I should have asked why you did that - you're right, consistency first. Thanks for pointing that out.

[delucis]

Ok, great. I’ll work on some documentation now.

[delucis]

@erquhart Ok, tried to document this. Would appreciate a few people reading through it, because it’s a little tricky to explain all the parts of this option.

@verythorough Do you have any thoughts?

[verythorough]

Sorry for the delay! I love how thorough these docs are. :)

I made one comment about the initial heading & description. Otherwise, looks good to me!

[verythorough]

To me, "customize commit messages" implies editing individual messages, rather than the templates that generate them. I think it might be clearer if the above two lines become something more like:

Commit Message Templates

You can customize the templates used by Netlify CMS to generate commit messages by setting the commit_messages option under backend in your config.yml.

[delucis]

Awesome! 😄