Add ability to track fund spending

[yuriytkach]

Add support with special slack command to be able to track spending of fund.

Command to support:
spend 1500 uah 2022-05-29 11:14 "Buy the drone", where

• spend - command name (required)
• 1500 - amount (required)
• uah - currency (optional, use fund's currency by default)
• 2022-05-29 - date (optional, use today's date by default)
• 11:14 - time (optional, use current time by default)
• "Buy the drone" - description (optional, put something by default)

Tracking of spending should go into separate table.

Add field to Fund object called spent to track total amount that was spent. Update that field each time a spending is recorded.

Return total spent amount when processing list command.

Return all spending records together with donations records when processing list <fundname> command.