level editor: character change does not persist after save

[evemartin]

Describe the bug When a level is saved with a non-van character, going back to edit the level shows the van again instead of the other character.

To Reproduce Steps to reproduce the behaviour:

1. Go to Level Editor and make a new map
2. Go to the Character tab and change the character to someone other than the van (i.e. Kirsty)
3. Save the level
4. When the popup appears asking if you'd like to edit or play, click edit
5. Go back to the Character tab - see that the character pictured is the van and the dropdown is empty. This also happens if you save the level and go back to it later to edit

Expected behaviour When returning to a created level to edit it, the character should be the one you set when you created the level

Screenshots What you see when you return to a created level with a non-van character to edit:

Additional context This might be related to the bug from task #1621

Good places to start

• Level editor template: https://github.com/ocadotechnology/rapid-router/blob/master/game/templates/game/level_editor.html
• Level editor view: https://github.com/ocadotechnology/rapid-router/blob/master/game/views/level_editor.py