Restructure store variables

[mvandenburgh]

Accessing the current dandiset from the vuex store requires the following:

store.state.dandiset.dandiset.dandiset

Breakdown of what each property means:

store
state
dandiset => the vuex namespace module
dandiset => the variable in the store that is of type `Version`
dandiset => the `Version.dandiset` property

We may want to consider one or both of the following:

1. Remove the dandiset vuex module and just make everything top level. We have no other modules so this makes sense to me.
2. Rename the store variable to version.

Applying both of the above would change the previous statement to store.state.version.dandiset.

[mvandenburgh]

This is no longer relevant now that we've moved from vuex to pinia #1265