Change activeKeyword-based logic to openSidebarSections-based logic

[maartenleenders]

Summary

This PR can be summarized in the following changelog entry:

• Makes sure you can open multiple keyword sections at the same time.

Relevant technical choices:

• Previously activeKeyword kept track of the keyword tab that was opened. We now opted for an array that keeps track of all open sections, which can be multiple.

Test instructions

This PR can be tested by following these steps:

• Checkout branch and run yarn install and yarn test (when the tests have passed CR you can assume they are fine, though do check them).
• You could expose the store and action to the window in edit.js, and dispatch (store.dispatch( action() ) and see whether the state was changed correctly.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

[maartenleenders]

Closing in favor of #9381