Make vue.js use relative paths for http resources instead of use absolute paths

Ref #312 (closed) and #347 (closed)

Move the slash char to the BASE_URL variable, so that the production value does not have a head slash, making it relative instead of absolute.

Also configure vue.js to use relative paths instead of absolute ones for static resources.

Instructions for how to test it can be found as a comment at !360 (comment 466124118)