[mediaqueries-5][editorial] Fixed a bunch of Bikeshed issues

[SebastianZ]

The spec. had a lot of Bikeshed issues, which I tried to fix with this patch.

What this change fixes:

• Descriptor references (Now all of them are pointing to the definition. Before it was some mix. Let me know if that's ok.)
• Fixed external references
• Indentation (tabs everywhere)
• Section IDs (obviously a copy & paste error)
• Merged redundant Privacy and Security Considerations sections

Notes:
There's still one link error left. "joint session history" refers to the HTML spec., but there is no definition for that. There's only one occurrence of that expression in an example. Not sure if that's what is meant.
There are two notes left regarding unexported definitions for the 'change' event and the 'update steps' algorithm. What should be done with them?

Sebastian