9845 group backlog tix by estimated completion

[fluxusfrequency]

What's this PR do?

• Groups tickets in the backlog based on the current predicted velocity when sorting by priority.
• Sets the default velocity to 10 if it comes back from the server as less than 1 (MVP behavior)

  Any background context or notes you want to provide?

The way the chunking works is a little complex, so I wrote a lot of comments. Open to other suggestions of how to make it clear what's going on there if the comments are too much.

Fixes #9869 as well.

Which code should the reviewer start with?

app/views/components/item-column/sprint.js

How is this feature tested (either manually or automatically)?

Load up Kanban and change the backlog sort to priority. Use the accordions to show and hide sprints. Try resizing some tickets and see what happens.

What are the relevant tickets (please include link(s))?

https://sprint.ly/product/1/item/9845 https://sprint.ly/product/1/item/9869

Screenshots (if appropriate)

What gif best describes this PR or how it makes you feel?

Definition of Done:

• [X] Is there appropriate test coverage?
• [ ] Does this PR require a regression test? All hotfixes require a regression test.
• [ ] Does this add new dependencies? If so, what are they?

[wookiehangover]

Looks like we broke the reorder buttons when Items are in sprints

[fluxusfrequency]

Now closes #9846 as well. Adds a column summary to current when sorting by priority. See this screenshot:

[wookiehangover]

Might not be the latest, but it looks like we borked the "gear" buttons on expanded cards.

[fluxusfrequency]

I'll take a look.

[fluxusfrequency]

@wookiehangover: fixed.