config: start adding path parameters

to change the urls of the admin page, the my account page and of
invites. Seems to work, but need to check all the code over and test.

models.go

@@ -467,3 +467,18 @@ type ContactMethodKey struct {
 	PIN      string
 	User     ContactMethodUser
 }
+
+type PagePaths struct {
+	// The base subfolder the app is hosted on.
+	Base string `json:"Base"`
+	// Those for other pages
+	Admin     string `json:"Admin"`
+	MyAccount string `json:"MyAccount"`
+	Form      string `json:"Form"`
+}
+
+type PagePathsDTO struct {
+	PagePaths
+	// The subdirectory this bit of the app is hosted on (e.g. admin is usually on "/", myacc is usually on "/my/account")
+	Current string `json:"Current"`
+}