Ability to "load" new 3.0 project

[benjiwheeler]

user should be able to do File -> New to load a new default 3.0 project in the editor, which replaces the current project data.

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Moved from scratchfoundation/scratch-www#2019
/cc @benjiwheeler @colbygk

[DeleteThisAcount]

i like

[benjiwheeler]

Unclear how to handle copy/duplicate and remix modes of creating a new project. (These are distinct from File->New, which just uses default project.)

File->New should create using default project, and immediately save.

(Maybe beyond the scope of this issue): File->Save as a Copy should show if you're looking at your project; when someone else's, that should become File->Remix.

[benjiwheeler]

Checklist of aspects of this to confirm before closing:

• Back button functionality
• Standalone playground player moves smoothly between hash ID, new project, upload and download states
• Standalone playground player url removes hash correctly on new project creation
• Standalone playground player correctly transitions to display project with ID when a hash is added, without reloading the entire webpage
• embedded player in www loads correctly in project page
• embedded player in www loads correctly when transitioned from project page to editor page
• embedded player in www loads correctly when loaded directly into editor page
• embedded player in www loads correctly when transitioned from editor page to project page
• Repeated transitions do not cause projects to load blankly
• embedded player in www transitions correctly to uploaded project state content
• embedded player in www transitions correctly to uploaded project state URL (clearing project id)
• embedded player in www transitions correctly to new project content
• embedded player in www transitions correctly to new project state URL (clearing project id)
• Did we always fetch project anew from storage on every transition back and forth between project page and editor page? I didn't introduce new functionality there, correct?
• revise existing tests that have been invalidated by changes
• add new tests