Reduce CSS stylesheet and JavaScript code

[lorenzschmid]

In 9ea7bf0 PurgeCSS and Terser have been added to the workflow example to improve the size of the included code (see also #5). Unfortunately, this leads currently to the following problems:

• Multiple leaflet CSS classes were wrongly removed
• JavaScript code could not be effectively mangled without introducing errors

Therefore, both tools have been removed again from the workflow examples in d887b95. In order to reinstate see this patch highlighting the removed parts. For PurgeCSS, the leaflet classes should be put on a whitelist (see also current bug in purgecss#513). For both, the installed node package version should be specified.

Alternatively, it might be worth to split up the single JavaScript file into multiple ones and import them only selectively (e.g. the leaflet module is only needed on the location page). Also, not all Bootstrap components are used and thus have to be imported.