Better safeguards against accidentable enabling of serial numbering on receipts

Created by: mlutfy

By default, cdntaxreceipts generates ID numbers based on the Contribution ID. This is the recommended mode. Some accountants do not like the gaps in receipt numbers (since not all contributions have charity receipts), so there is also an option for having a separate serial number.

PR #102 fixed a bug where this setting was being enabled by default. This can cause serious issues because it resets the serial numbering back to 1, therefore causing duplicate receipts numbers.

How can we make this more reliable? Some ideas:

• When serial_receipts are enabled, set the AUTO_INCREMENT to the max(contribution_id) + 1.
• Have more strong wording on the settings UI, so that admins know that they cannot go back once it has been changed.
• Freeze the setting once it has been set.
• Hide the setting from the admin interface? (document it in the README, how to enable via the Setting API)

cc @samuelsov