Adds Annotatable XBlock extracting from edx-platform repo

[irtazaakram]

Adds annotatable xblock extracting from edx-platform repo

Test PR

Make a test PR on edx-platform like this to run test cases on under development PR.

File references:

• xblocks_contrib/annotatable/annotatable.py has been extracted from
• xblocks_contrib/annotatable/templates/annotatable.html has been extracted from
• xblocks_contrib/annotatable/templates/annotatable_editor.html has been extracted from
• xblocks_contrib/annotatable/static/js/src/annotatable.js has been extracted from
• xblocks_contrib/annotatable/static/js/src/annotatable_editor.js has been extracted from
• xblocks_contrib/annotatable/static/css/annotatable.css has been extracted from
• xblocks_contrib/annotatable/static/css/annotatable_editor.css has been extracted from
• xblocks_contrib/annotatable/tests/test_annotatable.py has been extracted from

Acceptance Criteria:

• Extracted XBlock user experience should behave normal if required global css variables donesn't exist
• Extracted XBlock funcationality should work fine after turning its relevant Django Settings Flag
• In existing built in xblock implementation of edx-platform, update the status or data of the XBlocks. Then change the Django Settings flag and verify after shifting to extracted one, XBlocks are maintaining their required states (including XBlock completion status).
• Also test the vice versa of above step, change XBlock from Extracted one to Built in implementation to check fallback case testings.
• i18n is supported in the XBlock
• Create a course, add multiple XBlocks in the course. Keep different XBlocks in diversity of states (done/un-done etc). Export the course then validate the XBlock states after importing it again.
• Make a test PR for the extracted PR like this and fix the test cases if required.
• Write further unit test cases for good code coverage

Relevant Issue: openedx/openedx-platform#34841

[farhan]

@irtazaakram PR looks good now. Just make sure to:

Test and verify all the tasks mentioned in the PR description.
Fix the corresponding issue in the edx-platform as well.

[farhan]

Good work ⭐