Move contrast checking into Python

Change banner size dependent on group content
Tidy up inline JS for some files
Correct spelling

gallery/utils/contrast.py

@@ -0,0 +1,18 @@
+def contrast(background, light, dark, threshold = 0.179):
+    """
+    Calculate the contrast between two colors
+    background: tuple of (r, g, b) values
+    light: color to use if the background is light
+    dark: color to use if the background is dark
+    threshold: the threshold to use for determining lightness, the default is w3 recommended
+    """
+    r = background[0]
+    g = background[1]
+    b = background[2]
+
+    # Calculate contrast
+    uicolors = [r / 255, g / 255, b / 255]
+    c = [col / 12.92 if col <= 0.03928 else ((col + 0.055) / 1.055) ** 2.4 for col in uicolors]
+    l = (0.2126 * c[0]) + (0.7152 * c[1]) + (0.0722 * c[2])
+    
+    return light if l > threshold else dark