DB: Extract ApplicantSplitter class

[ottlinger]

with all helper methods to split data based on attribute values, e.g.

• [ ] by country and count
• [x] by gender
• [ ] by isFlexible
• [ ] by week
• [x] by room category

in

• [x] ChartHelper
• [x] ApplicationDatabaseReader
• [x] RoomDatabaseWriter

TODO: improve test coverage of ApplicantStateChanger and PaymentMailer