feat(input): support constant expressions for ngTrueValue/ngFalseValue

ngTrueValue and ngFalseValue now support parsed expressions which the parser determines to be constant values.

BREAKING CHANGE:

Previously, these attributes would always be treated as strings. However, they are now parsed as
expressions, and will throw if an expression is non-constant.

To convert non-constant strings into constant expressions, simply wrap them in an extra pair of quotes, like so:

    <input type="checkbox" ng-model="..." ng-true-value="'truthyValue'">

Closes #8041
Closes #5346
Closes #1199

docs/content/error/ngModel/constexpr.ngdoc

@@ -0,0 +1,21 @@
+@ngdoc error
+@name ngModel:constexpr
+@fullName Non-Constant Expression
+@description
+
+Some attributes used in conjunction with ngModel (such as ngTrueValue or ngFalseValue) will only
+accept constant expressions.
+
+Examples using constant expressions include:
+
+```
+<input type="checkbox" ng-model="..." ng-true-value="'truthyValue'">
+<input type="checkbox" ng-model="..." ng-false-value="0">
+```
+
+Examples of non-constant expressions include:
+
+```
+<input type="checkbox" ng-model="..." ng-true-value="someValue">
+<input type="checkbox" ng-model="..." ng-false-value="{foo: someScopeValue}">
+```