Adding paging to "Your recent attempts" tab (#40)

on challenge page.
Merge remote-tracking branch 'upstream/master'
Fixing pep8 warnings.
Reacting to code review, part 1:

various small edits.
Reacting to code review - adjusting stack

for recent attempts paging
Reacting to code review for #40 - adding AJAX

when "Your recent attempts" tab gets loaded.

Solves #117 as well.
Merge remote-tracking branch 'upstream/master'
Reacting to code review on #40 - minor changes.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/templates/solve_a_challenge.html
Merge pull request #129 from egor83/master

Added paging to "Your recent submissions" tab (issue #40)