Migrate web_fragments into XBlock

[Kelketek]

Story

"As a maintainer, I want a smaller footprint so that my effort can be more focused."

Full description

We have determined that web_fragments does not need to be an independent repository and can be moved back into XBlock.

Move the code for web_fragments into XBlock as a co-residing package, and update the release notes to indicate this change.

Completion criteria

• web_fragments is merged into XBlock as an additional package
• The release notes are updated
• Repos in the openedx org are updated to use the new version of XBlock and not depend on web_fragments

Documentation updates & improvements criteria

• The release notes are updated
• The web_fragments repository is archived, with a note in the README about its fate.

Relevant repositories

• https://github.com/openedx/web-fragments
• https://github.com/openedx/xblock