Warn user when calling subscribe outside of a reactive context

Calling subscribe outside of a reactive context will cache the
subscription, but there will be no ratoms around to remove the
subscription from the cache when the ratom is disposed. This can lead
to memory leaks where subscriptions stay in the cache indefinately.

This should fix issue #740

src/re_frame/interop.clj

@@ -91,3 +91,7 @@
   "Doesn't make sense in a Clojure context currently."
   [reactive-val]
   "rx-clj")
+
+(defn reactive?
+  []
+  true)

src/re_frame/interop.cljs

@@ -72,3 +72,7 @@
            reagent.ratom/Track "tr"
            "other")
          (hash reactive-val))))
+
+(defn reactive?
+  []
+  (reagent.ratom/reactive?))

src/re_frame/subs.cljc

@@ -1,7 +1,7 @@
 (ns re-frame.subs
  (:require
    [re-frame.db        :refer [app-db]]
-   [re-frame.interop   :refer [add-on-dispose! debug-enabled? make-reaction ratom? deref? dispose! reagent-id]]
+   [re-frame.interop   :refer [add-on-dispose! debug-enabled? make-reaction ratom? deref? dispose! reagent-id reactive?]]
    [re-frame.loggers   :refer [console]]
    [re-frame.utils     :refer [first-in-vector]]
    [re-frame.registrar :refer [get-handler clear-handlers register-handler]]
@@ -61,11 +61,19 @@
   ([query-v dyn-v]
    (get @query->reaction [query-v dyn-v])))
 
-
 ;; -- subscribe ---------------------------------------------------------------
 
+(defn warn-when-not-reactive
+  []
+  (when (and debug-enabled? (not (reactive?)))
+    (console :warn
+             "re-frame: Subscribe was called outside of a reactive context.\n"
+             "See: https://day8.github.io/re-frame/FAQs/UseASubscriptionInAJsEvent/\n"
+             "https://day8.github.io/re-frame/FAQs/UseASubscriptionInAnEventHandler/")))
+
 (defn subscribe
   ([query]
+   (warn-when-not-reactive)
    (trace/with-trace {:operation (first-in-vector query)
                       :op-type   :sub/create
                       :tags      {:query-v query}}
@@ -84,6 +92,7 @@
            (cache-and-return query [] (handler-fn app-db query)))))))
 
   ([query dynv]
+   (warn-when-not-reactive)
    (trace/with-trace {:operation (first-in-vector query)
                       :op-type   :sub/create
                       :tags      {:query-v query