fix: remove uid from scenes

[rostyslavvnahornyi]

#issue260
Hello! @kfarr, this pr includes issue#247, issue#248, issue#260.

P.S.: if you have got old scenes in firestore, delete them or just delete uid fields.

[kfarr]

Here are some WIP notes:

• We use the built-in firestore ID field but I added back using UUID as the actual ID, working great
• There was an issue related to author permissions that I resolved. @rostyslavvnahornyi I'd suggest that you use the same firestore security rules as here on your development database so that you can catch these errors: https://github.com/3DStreet/3dstreet-editor/blob/feature-firebase-save-load/public/firestore.rules
• The new timestamp fields are generated correctly when saving a new file
• entity?.object3D?.traverse((node) => { adding the "?" operator -- chat gpt tells me this code change will check for existence of entity and object3d before attempting to traverse. I might revert to original so that we get the actual error and can fix the root cause instead of silently ignoring this way
• Save (existing file) appears to work as expected

To do before merging:

• remove logic to open new window when saving scene
• make different notification success messages for creating newly saved doc vs saving existing doc
• errors captured from try/except in uploadScene (scene.js) should be exposed as notification message as well as console message
• try to "break" the hash logic -- see what happens with invalid uuid passed, for example
• NOT DOING, will let it break instead - update dev DB to retain some of the existing test scenes in new schema format (not doing)
• new issue for later, but ok to let go for this PR - as user who opens a scene, then opens sidebar menu, save as.. is used instead of save #275
• confirm "save as" ticket given above