In discoplace template, include splash but not persona (bug 1046577)

We don't need auth in discoplace, but we do have a splashscreen
while the content is loading.
Merge pull request #2378 from diox/commonplace-tweaks-for-discoplace

In discoplace template, include splash but not persona (bug 1046577)
Update ES_HOSTS to elasticsearch 1.2 endpoint.
Set MARKETPLACE_URL envvar in Dockerfile
Merge pull request #2387 from muffinresearch/set-env-var-for-mkt-url

Set MARKETPLACE_URL envvar in Dockerfile
Enable ES tests by default
Bump sphinx version to fix readthedocs issues (bug 1050811)
Merge pull request #2388 from diox/update-sphinx

Bump sphinx version to fix readthedocs issues (bug 1050811)
Fixed search tests
Log user to debug app install problems
Start ES after plugin install
Don't 500 when disabling apps with deleted versions (bug 1050938)
2.6 and 2.7 versions
Fix tests
Travis, meet nose-nicedots
........
allow image upload via external URLs in feed element create APIs (e.g., aviary)
Putting back link to Submit A New App (bug 1042942)

2-space indent
use drf exception to return 400s in ImageURLField
Merge pull request #2352 from dethe/relink_new_app

Putting back link to Submit A New App (bug 1042942)
Merge pull request #2391 from ngokevin/aviary

allow image upload via external URLs in feed element create APIs