Introduce destroy group function
Remove obsolete assignment
Rename insertAsset to createAsset
Introduce function to insert assets into asset map
Introduce function to get assets from asset map
Introduce AssetsModel
Trigger assetGroup:destroyed event
Define group re-rendering as one function
Re-render group when a group is destroyed
Introduce getter and setter for validity periods
Add missing semicolon
Move current validity period to assets model
Load AssetsModel.js in integration tests
Trigger validityPeriod:changed from AssetsModel