Add mirror page and top banner regarding primary mirror outage

[haarg]

Adds an about page with information about using MetaCPAN as a mirror, and replaces the 'ads' banner with an alert regarding the current outage.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.71%. Comparing base (cd37fc8) to head (ba79d27).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3446      +/-   ##
==========================================
+ Coverage   73.70%   73.71%   +0.01%     
==========================================
  Files          67       67              
  Lines        2354     2355       +1     
  Branches      333      333              
==========================================
+ Hits         1735     1736       +1     
  Misses        496      496              
  Partials      123      123              

Files with missing lines	Coverage Δ
lib/MetaCPAN/Web/Controller/About.pm	64.70% <100.00%> (+1.06%)	⬆️

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