Merge Account into other Account

[morremeyer]

Is your feature request related to a problem? Please describe.
Sometimes, users (read: me) accidentally create a duplicate account for one that already exists.

Currently, to get rid of the duplicate, all transactions have to be manually updated to the other account and the duplicate deleted.

Describe the solution you'd like
Add the possibility to merge one account into another, automatically updating all transactions it is involved in.

This feature should check that both accounts are of the same type (internal/external) and budget influence (on/off budget) to not screw up the budget.