level editor: character change does not persist after road is randomly generated

[evemartin]

Describe the bug
In some cases, even after selecting a different character in the Level Editor, when the level is saved and played it displays the default Van character.

To Reproduce

1. Log in as any user and open the Level Editor
2. Go to the Character tab and select a character other than Van

3. Go to the Random tab and randomly generate a map

4. (at this point, if you go back to the Character tab, the image will still be of the new character you chose but the dropdown value will be blank)
5. Save the level and, when prompted, choose to play the level
6. The character will be the Van and not the new character you chose

Expected behaviour
The level editor should remember the character choice even when recreating a new random road.

Once this has been fixed, please also check if changing other settings through the level editor also overwrites the character setting.

Good places to start

It seems like the function to generate the random road might be resetting the value of the character to the default, i.e. the van.

random_road.py: https://github.com/ocadotechnology/rapid-router/blob/master/game/random_road.py