Persist custom fulcrum servers

A custom fulcrum server addition should persist between refreshes or app restarts (ie stored like other wallet data/state).

This means the currently set custom server should remain, but also that previously used Fulcrum servers which successfully connect join the dropdown list of available [custom] fulcrum servers (perhaps marked as custom, and removable somehow).

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information