Migrate the `BP_Core_Friends_Widget` Legacy Widget as a Widget Block

[imath]

Merge the BP Blocks plugin's 'bp/friends' Block into Core.

[imath]

In 13002:

Merge the BP Blocks plugin's 'bp/friends' Block

• introduce the bp_get_dynamic_template_part() function which let you use a JavaScript template inside PHP thanks to the bp_core_replace_tokens_in_text() function.
• Adds a new assets/widgets/friends.php JavaScript template to our two template packs.
• Adds a new Core JavaScript file which acts like a parent class to all dynamic widgets front-end scripts (friends, groups, members).
• Add this front-end script to the Friends component.
• Adapt Grunt sass tasks.
• Add the Block JavaScript source files into src/js/bp-friends/js/blocks.
• Add the Block Scss source file into src/bp-friends/sass.
• Generate the development files to ease testing.

Fixes #8521