Very long, CPU-intensive process after asking to import selected Khan Academy resources
### Observed behavior

Need more info on this, but this is the best I can produce right now.. observe the 566m of CPU usage:

```
top - 01:39:58 up 2 days, 11:30,  1 user,  load average: 2.10, 2.17, 2.17
Tasks: 287 total,   1 running, 286 sleeping,   0 stopped,   0 zombie
%Cpu(s): 45.6 us, 12.9 sy,  0.0 ni, 41.5 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem :  8054696 total,   904492 free,  5504828 used,  1645376 buff/cache
KiB Swap: 16381948 total, 14482384 free,  1899564 used.  1499508 avail Mem 

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND                                                                            
15123 kolibri   20   0 3474360 1.112g  10440 S 201.0 14.5 566:35.82 kolibri                         
```

### Steps to reproduce

Not sure, but I have asked to download a select sub-part of the KA resources, and my laptop is stuck for hours.


### Context

Kolibri 0.9.0, debian installation source, Ubuntu 16.04

diff --git a/kolibri/content/management/commands/importcontent.py b/kolibri/content/management/commands/importcontent.py
index 8964fe70ae..f5be4ab783 100644
--- a/kolibri/content/management/commands/importcontent.py
+++ b/kolibri/content/management/commands/importcontent.py
@@ -67,7 +67,7 @@ class Command(AsyncCommand):
         parser.add_argument(
             "--include-unrenderable-content",
             action='store_false',
-            default=True,
+            default=False,
             dest="renderable_only",
             help="Import all content, not just that which this Kolibri instance can render"
         )
['kolibri/content/management/commands/importcontent.py']