WIP Add navbar problem page

[lucyleeow]

closes #400
Adds navbar to the top of problem page

[codecov]

Codecov Report

Merging #405 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #405   +/-   ##
=======================================
  Coverage   92.64%   92.64%           
=======================================
  Files          99       99           
  Lines        7943     7943           
=======================================
  Hits         7359     7359           
  Misses        584      584

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3e587e7...d1201da. Read the comment docs.

[lucyleeow]

The navbar in the event.html and problem.html pages are now the same length as the notebook iframe below it (and also the same width across the 2 pages).

Add navbar with 'description' and 'download kit' buttons to problem.html page:

Still to add leaderboard button linking to combined leaderboard for all events, with solutions deactivated unless it is the user's own solution.
I will work on this last part once @glemaitre is back.