[#1405] Migrate existing class saves/skills data to TraitAdvancement

[arbron]

Migrates existing entries from the saves and skills sections on class items into TraitAdvancement.

[Fyorl]

Some additional considerations:

1. We should consider doing the trait migration as a full migration to ensure the data gets persisted. In-memory migrations are fine for changing names or types, but since we're writing full advancement objects I'd feel more comfortable doing it as a full migration.
2. I think we can also strip out some code related to configuring skills and saves from ItemSheet5e, and potentially also trim some functionality from TraitSelector, though I'm less sure about that.

[Fyorl]

Thanks for making those changes, Jeff. I think this is a good opportunity to build up some framework for persisting source migrations that we can make more use of in the future.

Two things I want to make absolutely sure of when testing this:

• If the migration is run a second time, items do not repeat the 'persist source migration' workflow with diff: false.
• After the migration is performed, no items are left with dnd5e.needsMigration flags.

[Fyorl]

Looking good, Jeff, just a couple of things flagged. I have one issue raised in my testing that I hope you can reproduce:

1. (on 2.4.x branch) Create a new world
2. Create a new actor
3. Create a new class on that actor
4. Edit the class and give it any advancement (I used HitPointAdvancement)
5. Under its details tab, configure 2 saving throws and some skills (I used 6 skills)
6. Select 2 skills (also under the details tab)
7. Close down the world and switch to the trait-migration branch
8. Launch the world and run migrateWorld
9. Open the class item on the actor and select 'modify choices'
10. Advance to the skill trait flow
11. Observe no saved choices or choices to make

Also noticed a small display issue with the alignment of the icons in the advancement table. If you happen to know what the issue is there, feel free to make a quick fix directly. Otherwise I will take a look in due course.

[arbron]

Looking good, Jeff, just a couple of things flagged. I have one issue raised in my testing that I hope you can reproduce:

That is because this branch was created before TraitFlow was merged and never rebased, so it just displays the summary but doesn't actually apply anything.

Also noticed a small display issue with the alignment of the icons in the advancement table. If you happen to know what the issue is there, feel free to make a quick fix directly. Otherwise I will take a look in due course.

Not sure what you're referring to. Is it because the icons are aligned at the top with the title?

[Fyorl]

Not sure what you're referring to. Is it because the icons are aligned at the top with the title?

Just that they seem completely flush as to be touching the border of the element above them. I think it might just be a property of these particular icons though. I think others seem to have a bit of space baked in at the top so that I didn't notice it before.

[Fyorl]

Was able to do some further testing after the rebase, and it looks pretty good to me, thanks