Refactor 'hvStat.database.openIndexedDB'
Refactor 'hvStat.database.maintainObjectStores'
Increment database version and implement creation of new object stores
Implement storing of item/equipment drops
Implement display of equipment drops
Implement display of item drops

* Arrange the configuration of `ItemDrops` and `EquipmentDrops` in order
  to extend roles
Integrate the function of `Arena Rewards` tab into `Drops` tab
Remove 'Arena Rewards' tab
Swap the position of 'Drop PCT.' and 'Chance PCT.'
Fix following

* Fix the migration from version 5.5.5.1
* Fix bug of calls of `hvStat.statistics.drops.storeItem`
Implement a delegate class for indexeddb object stores

* Implement functions of export/import/delete `ItemDrops` and `EquipmentDrops`
Fix the display format of the date of equipment drops
Change the layout of 'Drops' tab

* Remove 'Clear Items' button and 'Clear Equipments' button
* Change some labels
* Modify 'Monster Database' tab

* Add export/import/delete UI for 'Item Drops' and 'Equipment Drops'
* Change element Ids
* Fix typos
Remove the event handlers of 'Clear Items' and 'Clear Equipments' on 'Drops' tab
Modify UI code for 'Monster Database' tab
Fix the processing of resetting Drops tab
Rename the tab name to 'Database' from 'Monster Database'
Modify the layout of Database tab and fix text representations
Refactor object store delegates