HUE-2072 [core] Adding support for HTTPS load-balancing

In order to support HTTPS load-balancing that is terminated at the load-balancer level, Hue needs to know that the request came through HTTPS. Django officially supports this by adding SECURE_PROXY_SSL_HEADER to the settings.py file and specifying the HTTP header (X-Forwarded-Protocol) to be set to https.

https://docs.djangoproject.com/en/1.4/ref/settings/#secure-proxy-ssl-header

At the load-balancer level you need to set that HTTP header. Here is the HAProxy example:

```
reqadd X-Forwarded-Protocol:\ https
```
HUE-2072 [core] Updating Proxy SSL support to be optional
HUE-2072 [core] Disable SSL proxy header by default

Add option in desktop/conf.py
Cleanup comments.
HUE-2080 [core] Update tours URLs

Updated to gethue.com and added additional content
HUE-296 [core] Add guppy debugger
HUE-296 [core] Add memory usage debug view

Add variable in settings to make it globally available.
[core] Fix LDAP and useradmin group list tests
HUE-296 [core] Memory profiling config
HUE-2026 [core] Create a doc model write permission

Add permission 'modify'.
Implicit read permission with 'modify'.
Modify pig and oozie apps to use 'can_modify' when
allowing other users to write.
HUE-2026 [core] Create a doc model write permission

Improved sharing modal UX
HUE-2026 [core] Create a doc model write permission

change modify to write