ENH: Update reorderItems() to use ORM where possible

[chrispenny]

Description

Closes #335

Based onto 3, but please feel free to update that with what you think is appropriate.

I'm not 100% familiar with all of the different Lists that we have, but I think it would only be the ManyManyList that we still need to process with SQL Queries, as their DB records have no representation in the ORM.

Additions

Just added a cheeky little extension point for onBeforeReorderItems, because "why not?".

[michalkleiner]

I'm all for simplification, though it seems suspicious that such a big removal will go without an impact in tests. Does it mean something significantly changed somewhere else to still support the reordering?

[chrispenny]

Hi @michalkleiner,

The module already supported sorting through the ORM, the only thing was that it only used the ORM for Versioned DataObjects. All I've done is change the criteria for when the module uses the ORM vs SQL Queries.

The relevant code for sorting through the ORM is still here:
https://github.com/symbiote/silverstripe-gridfieldextensions/pull/336/files#diff-ad5bbd1d14499e77c6ef3223c6c3746fa71333fec5831b4c3fcfa7a5e40f09a0R618

[chrispenny]

Hi @michalkleiner

We now have tests covering:

• has_many un-Versioned
• has_many subclass un-Versioned
• has_many subclass Versioned
• many_many un-Versioned
• many_many Versioned
• many_many with through model un-Versioned
• many_many with through model Versioned

Please let me know if there is anything else I can do to increase confidence in this change.

[GuySartorelli]

I'm slowly reviewing this inbetween other work.
If someone else has more time and wants to review it, please go ahead - but otherwise I'll get there eventually.

[GuySartorelli]

Looks good, works correctly locally, and the new tests are fantastic. Thanks for doing this.
Can you please rebase and resolve the merge conflict?

[chrispenny]

Thanks, @GuySartorelli! Rebase complete.

I'm not sure why, but running the workflow needs approval, if you could please 😃 .