feature - add flags to leaderboard

[dzuqe]

Description

I've added functionality to load flag icon next to the country in Community leaderboard

Refers/Fixes

#4152

Testing

Before: https://user-images.githubusercontent.com/21199234/55767457-a3ba4a80-5a96-11e9-8a29-7b1da9e1e30e.png

After:

1. Personally curated test data 4 countries: https://imgur.com/a/8UQcuSj

2. If the mapping of Country name to flag fails the default flag is USA e.g Incorrectly spelled Yemen displays US flag https://imgur.com/a/qMRwD15

3. Real Rinkeby data https://imgur.com/a/KhC84UK

I opted for the flag API instead of emojis as (from my personal research) Microsoft Windows doesn't support flag emojis. Idk, works on my Linux but the userbase of Linux compared to Windows is small.

[codecov]

Codecov Report

Merging #6509 into master will increase coverage by 0.03%.
The diff coverage is 42.10%.

@@            Coverage Diff             @@
##           master    #6509      +/-   ##
==========================================
+ Coverage   27.21%   27.25%   +0.03%     
==========================================
  Files         288      289       +1     
  Lines       26733    26752      +19     
  Branches     3943     3945       +2     
==========================================
+ Hits         7276     7290      +14     
- Misses      19190    19195       +5     
  Partials      267      267              

Impacted Files	Coverage Δ
app/marketing/views.py	11.92% <8.33%> (-0.10%)	⬇️
app/marketing/country_codes.py	100.00% <100.00%> (ø)
app/dashboard/embed.py	31.60% <0.00%> (+3.44%)	⬆️

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