Add ability to erase and modify shapes

[cleaaum]

This PR adds 2 features:

1. the ability to erase a shape
2. the ability to retain a modified shape.

[github-actions]

Staging application has been deployed and is available at: https://dash5-services.plotly.host/ml-exchange-staging
Production app name: ml-exchange
Current branch name: eraser-mode-clea
Commit: 05910b8

[hannahker]

Adding some notes here as I'm testing this out:

• [EDIT: This actually seems to happen in other cases where I try to export the annotations after deleting some of them, but I can't totally pinpoint what causes it.] Need to handle the case where someone deletes all the annotations via the keybind then tries to export the annotations. This is currently throwing the following error:

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   File "/workspace/callbacks/control_bar.py", line 688, in export_annotation

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     annotations.create_annotation_mask(sparse=EXPORT_AS_SPARSE)

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   File "/workspace/utils/annotations.py", line 86, in create_annotation_mask

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     image_height = slice_data[0]["image_shape"][0]

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 IndexError: list index out of range

• There seems to be a bug introduced with the "Annotated slices" dropdown. In the screenshot below, I annotated slice 1 and slice 238, but the dropdown has entries for other slices which don't have anything on them.

[hannahker]

Nice fixes! This works great on my testing. Can you just add the del keybind to the info popup in the bottom right? Then should be good to merge.