refactor: deposits

[weichweich]

fixes KILTprotocol/ticket/issues/2199, KILTprotocol/ticket/issues/2198

• Make the deposit calls less confusing.
• refactor the code to reduce duplication

Checklist:

• [ ] I have verified that the code works
  • [ ] No panics! (checked arithmetic ops, no indexing array[3] use get(3), ...)
• [ ] I have verified that the code is easy to understand
  • [ ] If not, I have left a well-balanced amount of inline comments
• [ ] I have left the code in a better state
• [ ] I have documented the changes (where applicable)