Discussion icon upgrade step

[erral]

• I signed and returned the Plone Contributor Agreement, and received and accepted an invitation to join a team in the Plone GitHub organization.
• I verified there aren't any other open pull requests for the same change.
• I followed the guidelines in Contributing to Plone.
• I successfully ran code quality checks on my changes locally.
• I successfully ran tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes.
• I included a change log entry in my commits.

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See: plone/plone.app.discussion#307

[mister-roboto]

@erral thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[erral]

mmmm this does not work, because p.a.discussion is a core-addon and may not be installed. Let's refactor this

[erral]

I have reimplemented the upgrade step as a python function, taking care of checking that all parts are available before setting the new icon.

[davisagli]

@erral Why are you adding this here instead of in plone.app.discussion? Upgrading Plone will also automatically run upgrade steps for the core add-ons listed in https://github.com/plone/Products.CMFPlone/blob/master/src/Products/CMFPlone/MigrationTool.py#L91, and plone.app.discussion is there. So I think it'd be tidier to keep the upgrade step there. (And then it will only run if the profile is installed.)

[erral]

My bad, I am really sorry. I looked indeed at plone.app.discussion to find upgrade steps there, but I didn't see them. I was looking for an upgrades folder but did not looked at it thoroughly because I thought that core packages' upgrades were here, that's why I added it here.

[davisagli]

@erral Not your fault! At some point in the past they were all here, then Maurits (I think) added the option to keep them in the individual package profiles. So it's not wrong here, but there's a better way now.