Add User count to CSV history stats

[heyman]

This PR does:

• Add a column "User count" to the history CSV stats (should fix More information in csv reports #1292)
• Makes the "Timestamp" column in history CSV the first column
• Remove the "Download response time history CSV" from Web UI since it has never worked, and fixing it is non-trivial (since we would either have to always store the whole stats history in memory, or always create a disk file somewhere, and I don't think we want to do either).

[codecov]

Codecov Report

Merging #1316 into master will decrease coverage by 0.31%.
The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master    #1316      +/-   ##
==========================================
- Coverage   81.04%   80.73%   -0.32%     
==========================================
  Files          24       24              
  Lines        2163     2154       -9     
  Branches      329      328       -1     
==========================================
- Hits         1753     1739      -14     
- Misses        327      330       +3     
- Partials       83       85       +2     

Impacted Files	Coverage Δ
locust/main.py	22.77% <0.00%> (ø)
locust/stats.py	88.70% <100.00%> (-0.03%)	⬇️
locust/web.py	88.73% <100.00%> (-0.61%)	⬇️
locust/contrib/fasthttp.py	87.70% <0.00%> (-1.07%)	⬇️
locust/runners.py	78.68% <0.00%> (-0.66%)	⬇️

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[cyberw]

LGTM!