Fixes Issue #216: Encourage individual icon component usage by removing Icon/index.js

[souviknsl07]

Description

As suggested in the issue, remove the Icon/index.js file to prevent engineers from importing and restructuring, also made the file structure such that -

//we end up with this:
import CheckmarkIcon from '@bufferapp/ui/Icons/Checkmark';
// instead of this
import CheckmarkIcon from '@bufferapp/ui/Icon/Icons/Checkmark';
// or this
import { Checkmark } from '@bufferapp/ui/Icons';

Due to this, had to change the imports for the icons in many other files and pathToIconComponents in scripts/generateIconComponents.js that's why the no. of files changed in this PR is huge, but it was a necessary change.

Motivation and Context

Fixes the Issue #216

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style and guidelines of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have updated the CHANGELOG document.
• I have added tests to cover my changes.
• I have performed a self-review of my own code
• I have tested my changes/additions in the latest Chrome, Firefox, and Safari.
• I have commented my code, particularly in hard-to-understand areas
• All new and existing tests passed.
• I have performed the accessibility audit of my UI changes according to the accessibility doc.
• [Buffer Engineers] Someone from the Design team reviewed and approved my changes
• [Buffer Engineers] I have notified the BDS team of my changes in the #proj-design-system Slack channel

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