Notify user when pool falls below threshold

[mitchhentges]

Every donation transaction includes semi-constant fees. When the pool amount falls far enough, then users will mostly be paying mining fees, rather than donations.

• Research good critiera for "low pool:
• Warn users when they have a "low pool:
  • Icon gets little exclamation point
  • On-click, show pop-up explaining situation, and that donations will not be made until pool is large enough again