Remove redundant schema file, Remove 'modified' prop from schema

[IshavSohal]

Related Item(s)

#491

Changes

• Removed the redundant StorylinesSchema.json file, since it was not being used anywhere else
• Prevented the slide-editor component from adding a modified prop to panels
• Removed the modified prop from the schema
• Called saveChanges() before executing main logic of panelModified() method, so that unsaved changes are captured and used when comparing the provided panel with its corresponding default config
• Fixed a few TS errors

Note

• I was not able to find any other temporary panel properties that break the schema, but let me know if I missed any

Testing

Steps:

1. Create a new product
2. Create a new slide
3. Change the type of a panel to a media panel (ex. Image, Video, Chart)
4. Notice there is no warning modal
5. Add some content to the panel (without saving)
6. Switch the type to another type, and observe the warning modal

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This change is 

[github-actions]

Your demo site is ready! 🚀 Visit it here: https://ramp4-pcar4.github.io/storylines-editor/issue-491