Disable TSDB by default (configurable)

[Mierdin]

There was a variable in config.go for a while that was supposed to get plumbed into an environment variable, but I haven't gotten around to it until now. We don't want TSDB exports to be enabled by default, because the error messages that come up when it fails to connect (which is totally normal unless you bother setting one up - not common for selfmedicate users) are alarming if you don't know what they mean.

So, this should take care of it - exporting to influxdb is off by default, and must be explicitly enabled.

[codecov]

Codecov Report

Merging #130 into master will increase coverage by <.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   31.74%   31.74%   +<.01%     
==========================================
  Files          20       20              
  Lines        2054     2060       +6     
==========================================
+ Hits          652      654       +2     
- Misses       1337     1340       +3     
- Partials       65       66       +1

Impacted Files	Coverage Δ
api/exp/resulthandlers.go	9.67% <0%> (-0.33%)	⬇️
api/exp/server.go	4.1% <0%> (-0.03%)	⬇️
config/config.go	61.45% <60%> (-0.5%)	⬇️

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