Move database code to frontend

[Tillerino]

Move as much of the persistence code as possible from the "backend" to the Tillerinobot "frontent" project. This should slim down the "Backend" classes in both the ppaddict and Tillerinobot frontend. It won't be possible to move everything, but a large part. Since I'll be using spring data repositories for the moved classes, this will take some time.

• [x] UserNameMapping - IRC name to user id mapping
• [x] GivenRecommendation - Recommendations history
• [x] BotUserData - Holds the content of UserData.
• [ ] BotUser - Saves the last visited version by IRC name (meh). Should probably be merged with BotUserData while we're at it.
• [x] ActualBeatmap - Saves the actual beatmap files as opposed to the meta data
• [ ] ...

[Tillerino]

UserNameMapping done in 03718fdef6e7ac023b116b4ce1a0ab5c3abe53bd