webapp: Add fallback "demo" when link nonexistent

Signed-off-by: Kumar Abhishek <abhishek@theembeddedkitchen.net>
webapp: Prefix local variables with 'var'

Signed-off-by: Kumar Abhishek <abhishek@theembeddedkitchen.net>
webapp: Add custom pin annotations

This allows the user to give a friendly name to pins e.g. P8_45 can be
named "SCK", P8_40 = "CS" and so on

Signed-off-by: Kumar Abhishek <abhishek@theembeddedkitchen.net>
webapp: Raise error and stop when no pin selected

for capture

Signed-off-by: Kumar Abhishek <abhishek@theembeddedkitchen.net>
webapp: whitespace cleanup

Signed-off-by: Kumar Abhishek <abhishek@theembeddedkitchen.net>