Use extracted HTML block base for About, StaticTab, CourseInfo

[kdmccormick]

Current state

xblocks-contrib has:

• class HtmlBlock

edx-platform has:

• class HtmlBlockMixin
• class _BuiltinHtmlBlock(HtmlBlockMixin)
• class CourseInfoBlock(HtmlBlockMixin)
• class AboutBlock(HtmlBlockMixin)
• class StaticTabBlock(HtmlBlockMixin)
• HtmlBlock = xblocks-contrib.HtmlBlock if extracted_flag else _BuiltinHtmlBlock

Desired state for rollout

xblocks-contrib has:

• class HtmlBlockMixin
• class HtmlBlock(HtmlBlockMixin)

edx-platform has:

• class _BuiltinHtmlBlockMixin
• class _BuiltinHtmlBlock(_BuiltinHtmlBlockMixin)
• HtmlBlockMixin = xblocks-contrib.HtmlBlockMixin if extracted_flag else _BuiltinHtmlBlockMixin
• class CourseInfoBlock(HtmlBlockMixin)
• class AboutBlock(HtmlBlockMixin)
• class StaticTabBlock(HtmlBlockMixin)
• HtmlBlock = xblocks-contrib.HtmlBlock if extracted_flag else _BuiltinHtmlBlock

Desired final state

xblock-contrib has:

• class HtmlBlockMixin
• class HtmlBlock(HtmlBlockMixin)

edx-platform has:

• class CourseInfoBlock(xblocks-contrib.HtmlBlockMixin)
• class AboutBlock(xblocks-contrib.HtmlBlockMixin)
• class StaticTabBlock(xblocks-contrib.HtmlBlockMixin)