Bug 1005114 - Profile includes list of completed tasks

User profile page displays task in progress and list of completed tasks, most recent first. The list of completed tasks is paginated. The "Welcome, user" string in the header links to the user profile. The profile page is only accessible by the user who owns it. Pagination is generalized with custom Jinja2 macro. 

Also proposing a fix for how `page_url` helper (and associated tests) generate URL strings used for pagination.

(For previous discussion see PR #120.)

@bobsilverberg r?