Implement support for task statement in HTML and MD

[wil93]

I tested adding (from AWS and from CLI) and removing (from AWS), and tested the rendering.

If you like this change, I think the only change needed is to write the DB updater (and "vendorize" the new JS dependencies, I didn't do it here because I didn't want to push big files).

• [ ] make sure that AWS supports uploading the new statement types
• [ ] fix cmsAddStatement not fetching assets

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This change is 

[CristianCantoro]

FWIW, we are already using Markdown for task statements. We using pandoc and a .tex template in the middle, so we generate the task statements separately with a Makefile.

[CristianCantoro]

Example statement in Markdown, with PDF output:

• testo.md
• testo.pdf
• template.tex
• Makefile

[stefano-maggiolo]

Let's try to merge #1067 first

[wil93]

Now that #1067 is merged, maybe this can be considered :smiley:

[codecov[bot]]

Codecov Report

Merging #1086 into master will decrease coverage by 19.89%. The diff coverage is 19.54%.

@@            Coverage Diff             @@
##           master    #1086      +/-   ##
==========================================
- Coverage   61.99%   42.09%   -19.9%     
==========================================
  Files         228      230       +2     
  Lines       16460    16711     +251     
==========================================
- Hits        10204     7035    -3169     
- Misses       6256     9676    +3420

Flag	Coverage Δ
#functionaltests	?
#unittests	42.09% <19.54%> (-1.07%)	:arrow_down:

Impacted Files	Coverage Δ
cms/server/admin/handlers/__init__.py	0% <ø> (-100%)	:arrow_down:
cms/server/contest/handlers/__init__.py	100% <ø> (ø)	:arrow_up:
cms/server/admin/handlers/task.py	0% <0%> (-41.02%)	:arrow_down:
cms/db/__init__.py	96.42% <100%> (ø)	:arrow_up:
cms/__init__.py	100% <100%> (ø)	:arrow_up:
cms/server/contest/handlers/task.py	31.25% <12.19%> (-12.23%)	:arrow_down:
cmscontrib/AddStatement.py	21.66% <20%> (-51.42%)	:arrow_down:
cmscontrib/loaders/italy_yaml.py	7.89% <8.33%> (-0.06%)	:arrow_down:
cms/db/task.py	73.15% <91.66%> (-8.6%)	:arrow_down:
cms/server/admin/__init__.py	0% <0%> (-100%)	:arrow_down:
... and 119 more

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