#171 do not display deleted questions/answers

This rule was only followed in the question list.
Now in the user's page, questions and answers with is_deleted=False are
omitted, and also in the question details, answers with is_deleted=False
are omitted (although deleting answers this way is not supported yet).
#171 show alert when viewing deleted question
#171 Show alert for deleted answer

(Although deleting answers is not possible yet)
Merge pull request #227 from danielhers/soft-delete

Fixes for soft delete