feat: add multiple house delivery

[evemartin]

Add the ability to add multiple houses for delivery in a created level, as well as the ability to delete houses.

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This change is 

[faucomte97]

Reviewed 4 of 5 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @evemartin)

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game/end_to_end_tests/test_level_editor.py line 28 at r2 (raw file):

        add_road_button.click()

Remove 2 of those empty lines please

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game/end_to_end_tests/test_level_editor.py line 48 at r2 (raw file):

        houses_after_delete = self.selenium.find_elements(By.CSS_SELECTOR, "rect[fill='#0000ff']")
        assert len(houses_after_delete) == 1

Could you also add a check, maybe, that when the level gets created it has the expected number of houses?
Ie from the backend Level object. Might need to be a different test though. Not 100% sure how feasible it is, if too difficult we can dismiss it.

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game/static/game/js/level_editor.js line 60 at r2 (raw file):

            id: 'delete_road',
        },
        MARK_DESTINATION_MODE: {

This mode is no longer needed if I understand correctly.
Please remove all references to it, including any leftover logic surrounding it (that you haven't updated for this task) as well as the SVG it uses.

[faucomte97]

Reviewed 7 of 7 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @evemartin)