Add global search functionality

Added the global search function. It's made up of two parts;

**1. Quick Search**
 - Allows the user to search for cards and boards from anywhere in cantas
 - Displays results for the first 4 boards and 3 cards
 - Pressing enter (or clicking the "View all results" button) while in the quick search view takes the user to the full results page

**2. Search Results Page**
 - Full page view of all matching cards and boards
 - Available via /search/{term}

**Notes**
 - I have made a slight change to the styling of cards and boards, happy to change back if you don't like it
 - Removed pagination using scroll, now uses a load more button (Less glitchy)