Add api/v1/stats/user/:username/rounds endpoints

[bpfoley]

Code borrowed from theme.inc show_tally_specific_stats

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Test hint edit from @cpeel courtesy of @bpfoley:

API_KEY=<api key here>
ROOT=https://www.pgdp.org/~bfoley/c.branch/user-stats-api
curl -X GET "$ROOT/api/v1/stats/user/bfoley/rounds" \
   -H "Accept: application/json" \
   -H "X-API-KEY: $API_KEY"

curl -X GET "$ROOT/api/v1/stats/user/bfoley/rounds/P1" \
    -H "Accept: application/json" \
    -H "X-API-KEY: $API_KEY"