NEW Add migration task from gorriecoe/silverstripe-link

[GuySartorelli]

Description

Adds a migration task and guide for migrating from gorriecoe/silverstripe-link and gorriecoe/silverstripe-linkfield

Requires silverstripe/silverstripe-framework#11182 for CI to go green.

Manual testing steps

• Set up some links with gorriecoe/silverstripe-link
  • be as creative as you want with it, hopefully the guide tells you how to deal with your scenario.
  • Ideally have a range of relations
    • has_one
    • has_many (with custom sort column)
    • many_many (with many_many_extraFields)
    • many_many through (with extra $db fields)
    • many_many through, with polymorphic has_one on the "from" end of the join model
• Use the upgrade guide. Where it says to composer require silverstripe/linkfield:^4 obviously you'll need to pull in this PR instead.

Issues

• Migration from gorriecoe #229

Pull request checklist

• The target branch is correct
  • See picking the right version
• All commits are relevant to the purpose of the PR (e.g. no debug statements, unrelated refactoring, or arbitrary linting)
  • Small amounts of additional linting are usually okay, but if it makes it hard to concentrate on the relevant changes, ask for the unrelated changes to be reverted, and submitted as a separate PR.
• The commit messages follow our commit message guidelines
• The PR follows our contribution guidelines
• Code changes follow our coding conventions
• This change is covered with tests (or tests aren't necessary for this change)
• Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• CI is green

[emteknetnz]

Tested locally, seems really good, only some minor things

[emteknetnz]

I think it would make sense to move the yml config up much higher and be above where the diff examples are, because when you run through this you've removed all of the old config by this point, yet with the yml you're asked to define it, though you have nothing to refer to

[GuySartorelli]

I'd prefer to keep all of the migration task configuration in one chunk - are you suggesting moving all of the migration task configuration up above the rest of the setup?

I'm alright with doing that (and I'll update the v2/v3 to v4 migration docs while I'm at it) if that's what you want, but just want to clarify before doing it.

[emteknetnz]

Yeah move the migration task config above the bit where it tells you to delete old stuff. Because you need to refer to that old stuff when adding your migration task config

[GuySartorelli]

Done.

I've also moved the "Take a backup of your database" into its own callout block to highlight that a bit better - it's easier to miss when its a step buried in a section like that, especially for experienced devs who may want to just skimread the docs and then figure it out themselves.