Small tweaks for adding a new application member:

- raise specific invitation type if invite not found in invitation domain classes
- more terse assignments of defaults in invitation service, smh
- terser margin expression for inline input fields
- sass formatting
- use translation for cancel link
- oxford comma for app team management permission explanation
- do not format environment roles with hyphens for role selection
- generalize some additional methods in the invitation domain base class
- use plain atst.models import path

styles/components/_forms.scss

@@ -180,10 +180,10 @@
 }
 
 .input__inline-fields  {
-  margin: 1rem 0 1rem 0;
+  margin: 1rem 0;
 
   &.input__inline-fields--indented {
-    margin-left: 4*$gap;
+    margin-left: $gap*4;
   }
 
   &> fieldset.usa-input__choices label {