Fix #2243 - AssetSize bar disappears when deleting an asset

[lindapaiste]

Fixes #2243

Should be merged after #2187 because I used Redux Toolkit for this.

Changes:

• Convert assets reducer to Redux Toolkit.
• When deleting a file, decrement the totalSize by the size of that file.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[lindapaiste]

@raclim I think this bug is worth fixing. If we want to hold off on redux-toolkit than I can write a different fix using the existing reducer. Should I do that?

[raclim]

@lindapaiste I think it would be great to get Redux Toolkit in! Would be down to have that in first and then this merged in!

[PiyushChandra17]

Nicely Done