Migrate the `BP_Core_Login_Widget` Legacy Widget as a Widget Block

[imath]

Merge the BP Blocks plugin's 'bp/login-form' Block into Core.

[imath]

In 12999:

Merge the BP Blocks plugin's 'bp/login-form' Block into Core

• Adapt Grunt sass task.
• Add the Block JavaScript source files into src/js/bp-core/js/blocks.
• Add the Block Scss source file into src/bp-core/sass/blocks.
• Generate the development files to ease testing.
• Update the BP_Core class to register this block and set its block globals.
• Update the BP_Component class so that using parent::setup_globals() from the BP_Core->setup_globals() method only sets the block_globals property.

Props dcavins

Fixes #8517