Make the image remove option work

[rtibbles]

Summary

Description of the change(s) you made

• Ensures that images get properly cleared when using the 'remove' option from the context menu
• Create remove event mechanism for clearing custom span elements for custom markdown fields.
• Consolidates all blank space wrapping in custom markdown field logic.
• Cleans up squire keydown handling.
• Modifies prop values using web component interface rather than directly modifying a component prop
• Tweak markdown conversion to prevent addition of excess line breaks, which would stop images and formulae from appearing inline with each other

Manual verification steps performed

1. Added an image to an exercise question.
2. Pressed close.
3. Reopened the question for editing, used the context menu to remove the image.
4. Used arrow keys to navigate around the editor window
5. Added line breaks and removed line breaks between images

Screenshots (if applicable)

Screencast.from.11-22-2022.02.28.17.PM.webm

Screencast.from.11-23-2022.02.31.18.PM.webm

References

Fixes #3817

[bjester]

I'm still noticing some differences with this method vs with the cursor next to the image and inputting backspace. It seems that this still leaves an image wrapper, and that becomes a pesty blank space that will only be removed if you explicitly select it with the cursor and input backspace, or close and reopen the exercise.

[rtibbles]

Oh, interesting - I guess the existing destroy wasn't doing enough. Will look in more detail!

[rtibbles]

So, I've managed to make the span delete itself, but the   on either side that both get deleted with a simple backspace are still not removed.

[rtibbles]

OK, this should all be working as intended now. I've updated the PR description with the additional fixes and an updated screencast.

[bjester]

LGTM!

[pcenov]

@rtibbles I was able to identify the following issues:

1. The images are not displayed in Kolibri once the channel has been imported there:
   
2. After I remove an image and then I click again in the editor the image reappears. Happens in both Chrome and Firefox:

2022-12-06_15-10-43.mp4

[rtibbles]

The images are not displayed in Kolibri once the channel has been imported there:

For this, how was this image added? Was it pasted into the text box, added using the 'add image' button?

After I remove an image and then I click again in the editor the image reappears. Happens in both Chrome and Firefox:

Interesting, this must be an issue with the data not being persisted properly.