Suggest donation amounts based on a Contact's giving history

Makes historical donation amounts array more flexible, and adds a hook for site builders to include other amounts for consideration by the donation suggestion algorithm.

Added some safety checks for historical donation amounts array, and the actual contact object - in practice either of these could be empty.

Changes:

• Adds an "Selection sort order" option to donation settings, to ensure that suggested amounts are sorted in a particular order. Default is unsorted, ie: in whatever order the suggestion method produces. This is useful if, for example, a client believes that sorting suggestions in *DECREASING* order might yield higher donations.
• Removes applicability of suggestions to RECURRING donations, so they are only offered for one-off donations. The recurring case had a small but easily noticeable bug, and the fact it hasn't been noticed suggests there isn't demand for suggestions on recurrings. It seems a poor idea to leave code around that isn't being used and tested. This could be revisited if there is demand.

Refactored function parameters, and added alter hook for suggestion config.

Fix operator for minimum check from > to >=