Add individuals pages for goals, targets and Indicators

[brunosan]

It makes easier to link (vote/comment?), also embed e.g. Discuss.js

Can probably be done with a template view that loads the appropriate goal/indicator/target via an URL parameter.
E.g.
http://sdg-data.github.io/web/targets/?target=4.6#
http://sdg-data.github.io/web/goals/?goal=4#
http://sdg-data.github.io/web/indicator/?indicator=x#