Migrate the `BP_Groups_Widget` Legacy Widget as a Widget Block

[imath]

Merge the BP Blocks plugin's 'bp/dynamic-groups' Block into Core.

[imath]

In 13004:

Merge the BP Blocks plugin's 'bp/dynamic-groups' Block

• Adds a new assets/widgets/dynamic-groups.php JavaScript template to our two template packs.
• Add a front-end script to the Groups component to manage Widget Block sort filters dynamically.
• Adapt Grunt sass tasks.
• Add the Block JavaScript source files into src/js/bp-groups/js/blocks.
• Add the Block Scss source file into src/bp-groups/sass.
• Generate the development files to ease testing.

Fixes #8523