Remove dynamically included Google Webfonts (GDPR) from offline.html

[SvenBrunk]

Removed inclusion of Google Webfonts directly from Google and replaced it by multiple alternatives: 1) still Source Sans Pro in case the browser has it from any other source 2) OXID font in case anyone uses it 3) Helvetica and Arial for 2 of the most widely used fonts installed on most devices 4) generic sans-serif (as before)

[CLAassistant]

All committers have signed the CLA.

[Sieg]

Hi @SvenBrunk, please sign the updated CLA.