#398 Upgrades jquery to version 3 and silences console outputs

As far as I tested #400 (merge to test) looked successful. @max.mehl could we coordinate the release of this PR? I want to do some checks after release, since it's more than just tiny text changes. --------------- There are some console warnings, errors and debug messages on the current page (see screenshot). This PR * Adds `data-cycle-log="false"` that silences the front page carousel output * Removes `hljs` stuff from `fsfe_head.xsl` since the highlight js isn't loaded there (leads to an error) * Removes the link to the missing source maps of the `jquery.cycle2.min.js` script And finally I upgraded to jquery 3: * I removed the previous jquery versions * Tested the supporter and event form - still working * Added jquery 3 min version and the source maps (!) we should test the important forms after release closes FSFE/fsfe-website#396