Dedupe Rules - don't allow multiple defaults for same type, don't allow removing sole strict rule for each type, improve UI

The admin interface for managing dedupe rules needs several fixes and improvements:
1. Prevent setting multiple rules for same contact type / rule type as default (see steps below for how that can happen now)
2. Prevent user from editing rules such that they remove the strict rule for a given contaact type (there always should be one default strict and one default fuzzy rule for each contact type). Throw a form rule is the edit breaks this business rule.
3. Add is_default chkbox to rule edit form to make it easier to see how to change default. Make sure the postProcess unsets other default rule of same type if needed. (We can leave the more -> action of Make Default in selector as alternate UI for changing default.)

---- Original Post
Multiple default dedupe rules for the same contact type and level may occur if a Default Fuzzy Rule is edited and changed to Strict - or vice-versa.

Steps to reproduce:

1. Consider the default dedupe rules.
2. Edit Individual Fuzzy.
3. Change Level from dropdown from Fuzzy to Strict and press Save.
4. Look at two Strict Defaults for Individuals.