Styling updates

[jessicasalmon]

• [x] add button to nav to toggle between home and about
• [x] control size of map and zoom level on mobile
• [x] style footer on mobile
• [x] general styling for mobile
• [x] render correct image for each building type on details view
• [x] responsiveness updates to cards: now show 3 on large screen
• [x] fix height of headers in image cards to ensure card size is kept consistent
• [x] display building name on cards if present, otherwise display street number and street name
• [x] create Banner component, which displays the text 'Explore locally listed buildings in Islington' on homepage

relates #36 #37 #38 #39 please review after #34

[sohilpandya]

@jessicasalmon can you please merge this in.