Bug 1032371 - Display users who have completed task on task detail page

@tessie I hate to have to say it, but the indenting for this is quite wrong. In html templates we use 2 spaces for an indent. In python files we use 4 spaces. In the section above you have also used 3 spaces and 1 space in different places, and the tags are not even all lined up. For example, the opening `{% if  %}` does not start in the same column as the closing `{% endif %}`. You should be able to see these inconsistencies in your editor, and if for some reason you cannot then you should be able to see them in the Github diff. For this reason I suggest you carefully review the Github diff for your PR *before* you submit the PR (Github provides you with a preview before you submit the PR) and look for indentation issues. That will save us from having to do all this back and forth about indentation.