Hide share and remix in levelbuilder

[cpirich]

• Add a new hide_share_and_remix property on levels that derive from Blockly (includes droplet levels such as gamelab and applab) and Weblab. It modifies the toolbar much like the recent change for embed levels. Exposed it in levelbuilder for all of those types of levels
• Fixed the way weblab generates its level_options to behave more like Blockly derived levels. In this case, it improves that handling of boolean level properties.
• Fix files API error when uploading files with unknown mime types (e.g. a file with no extension): see https://app.honeybadger.io/projects/3240/faults/31733196

[codecov-io]

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[islemaster]

LGTM. I wonder at what point we stop and find a way to refactor project-level features like this (for applab/gamelab/weblab) into some sort of module we can mix-in as needed.