Use the default fallback behavior of localforage

Since we store a compressed base64 string we can usie indexeddb on mobile when available.

This PR removes the the use of websal by default.

Tested with a 15km2 extent with 3 layers (pixelkarte, wander, cadastralwebmap), here the result:

Amdroid Chrome: 99mo,
               Firefox: 71mo
               Opera: 138mo

ios 8 Safari: 58 mo