Prepare documentation for 1.2 release

[aarranz]

Update documentation for final 1.2 release.

• Remove Django 1.8 references from the installation guide.
• Ensure mkdocs builds WireCloud documentation correctly.
• General documentation improvements.
• Improve authentication backend documentation on the IdM section. See Update doc #327 for more details.

[coveralls]

Coverage increased (+1.4%) to 80.373% when pulling 29a47aa on aarranz:fix/documentation-for-1.2 into 8b095cd on Wirecloud:develop.