[MER-2573] Task to create contained objectives for existing sections

[gastonabella]

MER-2573

This PR is the first of three parts to create and use the new ContainedObjectives table. Similar to ContainedPages, this works as a quick way of knowing which objectives are attached to activities within a specific container.

As part of this pull request:

• New contained_objectives table and new v25_migration field under sections that states if the contained objectives were built for a section.
• A new Mix Task is created for building all the contained objectives for the existing sections. You can run it with mix create_contained_objectives in the console.

The remaining two PRs will include the changes for automatically building the contained objectives when a section gets created, remixed, or updated, and the new Filter By implementation on the Learning Objectives tab under the Instructor Dashboard.

[augusto1024]

Amazing job 🤩!!! I just left a couple of small NITs/questions

As we discussed yesterday in the office, when I ran the task while running the app, the app crashed, so maybe we should take a look at that. I tested it with a couple more sections and works great!

[gastonabella]

Based on Augusto's comment and after some debugging I noticed an Oban weird error caused by having two instances of Oban running at the same time (server running locally and the Oban instance started by the mix task).
What is weird is that it only happens when running the server with an iex shell (via iex -S mix phx.server) and running the task at the same time. I think that should not be an issue in staging or prod, but I made the task accessible from iex via Mix.Tasks.CreateContainedObjectives.run_now() just in case.