Decay search improvements

• Each decay "card" now has a checkbox
• The stripping line pill changes colour with the card
• The warning pills are expanded, and their tooltips explain what they mean
• Added filtering by decay head (can only choose from the set of decay heads!)
• Added filtering by already-selected
• New component SelectParticle: essentially a wrapper for react-select that is populated with our particleProperties metadata
• Fixed minor bug in the number of new results loaded in the infinite scroller
• Display total number of results and how many are shown
• Filter decays by tag: Closes #15 (closed)
• Decay selection now persists across changes in filters