dcc redesign: refactor `Loading` component #453

Job	Run time
Build Dash Package	2m 57s
Detect Relevant Path Changes	6s
Typing Tests	2m 44s
Table Unit/Lint Tests (Python ${{ matrix.python-version }})	0s
Table Server Tests (Group ${{ matrix.test-group }})	0s
Run Background & Async Callback Tests (Python ${{ matrix.python-version }})	0s
	5m 47s

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