Limit all stats GraphQL endpoints by default to 1 year of stats

[jaywink]

The filter 'period' can be given to specify a custom range. It accepts
any integer + period key 'd' for days, 'm' for months and 'y' for years.

[codecov-io]

Codecov Report

Merging #153 into master will increase coverage by 0.19%.
The diff coverage is 45.45%.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
+ Coverage   61.54%   61.74%   +0.19%     
==========================================
  Files          29       29              
  Lines         775      792      +17     
==========================================
+ Hits          477      489      +12     
- Misses        298      303       +5

Impacted Files	Coverage Δ
thefederation/schema.py	60.54% <45.45%> (+1.01%)	⬆️
...n/templates/thefederation/social/platform_nodes.md
...n/templates/thefederation/social/platform_users.md	91.66% <0%> (ø)

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