giphy: Use GIPHY web SDK to allow inserting GIFs in compose box.

We use GIPHY web SDK to create popover containing GIFs in a
grid format. Simply clicking on the GIFs will insert the GIF in the compose
box.

We add GIPHY logo to compose box action icons which opens the GIPHY
picker popover containing GIFs with "Powered by GIPHY"
attribution.

zerver/lib/events.py

@@ -444,6 +444,18 @@ def fetch_initial_state_data(
     if want("video_calls"):
         state["has_zoom_token"] = settings_user.zoom_token is not None
 
+    if want("giphy"):
+        # Normally, it would be a nasty security bug to send a
+        # server's API key to end users. However, GIPHY's API key
+        # security model is precisely to do that; every service
+        # publishes its API key (and GIPHY's client-side JS libraries
+        # require the API key to work).  This security model makes
+        # sense because GIPHY API keys are all essentially equivalent
+        # in letting one search for GIFs; GIPHY only requires API keys
+        # to exist at all so that they can deactivate them in cases of
+        # abuse.
+        state["giphy_api_key"] = settings.GIPHY_API_KEY if settings.GIPHY_API_KEY else ""
+
     return state