light/dark mode switch improvements and bugfix in search index (#29)

* Display link to source code repo as image if params.source.iconLight is specified
dont use hardcoded "images/" prefix in params.logo.lightMode and .darkMode

* don't use javascript template string in search index because it causes a js ReferenceError if a page contains something like ${variableWhichDoesntExistInJs}

* fix bug where light/dark mode switch isn't in the right position when the user loads the site the first time with preferred dark mode
add a sun icon on the switch in light mode and a moon icon in dark mode (icons are drawn by me so no copyright issues)