Enable turning lazyloading on/off in settings.yml

[arthurattwell]

Some projects don't need (or shouldn't have) lazyloaded images. This change lets us toggle lazyloading on or off from settings.yml.

How this works:

• in bundle.js, we include a settings.js, which Jekyll populates with the lazyloading value from settings.yml.
• settings.js creates a settings object which is then available for all subsequent JS. (In future, we could add other settings to this object if we need them for JS. We do something similar with locales.js, except there we can safely include all of locales.yml, whereas settings.yml may include values that are only for site generation, and should not be exposed in client-side JS. So we are selective about which settings to include in the settings object.)
• lazyload.js, which is also included in bundle.js, checks whether lazyloading is activated (i.e. settings.web.images.lazyload === true). If it isn't, lazyload.js immediately lets all the images on the page load, without waiting for the image to scroll nearby (i.e. is skips the intersectionObserver and changes data-src attributes to src).