Stop using Vuex global content nodes state - “Edit modal” feature

Summary

When using the edit modal,

content nodes are added to contentNodesMap Vuex state:

https://github.com/learningequality/studio/blob/d6f06c273469ae8ad202774d40accc69817284ae/contentcuration/contentcuration/frontend/channelEdit/vuex/contentNode/index.js#L15

and content nodes ids to previousStepsMap/nextStepsMap:

https://github.com/learningequality/studio/blob/d6f06c273469ae8ad202774d40accc69817284ae/contentcuration/contentcuration/frontend/channelEdit/vuex/contentNode/index.js#L58

https://github.com/learningequality/studio/blob/d6f06c273469ae8ad202774d40accc69817284ae/contentcuration/contentcuration/frontend/channelEdit/vuex/contentNode/index.js#L45

Examples:

https://github.com/learningequality/studio/blob/d6f06c273469ae8ad202774d40accc69817284ae/contentcuration/contentcuration/frontend/channelEdit/components/edit/EditModal.vue#L333

https://github.com/learningequality/studio/blob/d6f06c273469ae8ad202774d40accc69817284ae/contentcuration/contentcuration/frontend/channelEdit/components/edit/EditModal.vue#L334

The goal of this issue is to refactor the feature to not add/retrieve data to/from contentNodesMap as well as to/from previousStepsMap/nextStepsMap but rather move towards using a more local state that will be cleared after a user stops using it.

Background

• We accumulate content nodes data in Vuex contentNode/state from various Studio features of the channelEdit app and there is no mechanism for clearing them which causes memory leaks. This issue is part of a larger group of issues (see https://github.com/learningequality/studio/issues/3363) that aim to refactor problematic features away from using Vuex global state towards private in-components state or state that’s shared between more components but is cleared at some point and optimized performance-wise in general.
• Ultimately, we want to get rid of globally stored content nodes data completely, however, this will be implemented incrementally, and therefore it’s fine to use mixed sources of data in the transition stage as long as all features continue working well from the user-point of view. Using composables is not required in all cases but is recommended as it has proven to be useful for state management across our products and it is flexible enough to allow us to keep using Vuex partially during the transition stage.

Acceptance criteria

• No content nodes are added/retrieved to/from contentNodesMap and previousStepsMap/nextStepsMap when using the edit modal (applies to all sub-components if there are any)
• The new implementation avoids the pitfalls of our current implementation that are described in the “Background” section and is optimized performance-wise overall

Blocked by

• https://github.com/learningequality/studio/issues/3468

Blocking

• https://github.com/learningequality/studio/issues/3469

Start with channelEdit/vuex/contentNode/index.js and the referenced usages in channelEdit/components/edit/EditModal.vue. Review the dependency on issue 3468 before tracing how the edit modal and its sub-components use contentNodesMap, previousStepsMap, and nextStepsMap. Done means the modal no longer reads or writes those global maps and its local state is cleared appropriately.