Cache dashboard, avoid running queries. (fix #3)

[dac514]

There is a new bin/cache.php file that can be setup as a cron job, it should be run like this:

wp eval-file bin/cache.php

This will store the rendered html into a transient, and will avoid running the SQL unnecessarily.

Other ways that we could optimize:

• The pressbooks_stats_exports could have millions of rows on pressbooks dot com, but the stats page doesn't look further than 4 months. We could delete every now and then?
• Maybe add MySQL indexes on wp_users:user_registered and wp_blogs:registered? I'd want to wait for New Relic to see if this is identified as a slow query first though.

[codecov[bot]]

Codecov Report

Merging #6 into dev will not change coverage. The diff coverage is 0%.

@@        Coverage Diff        @@
##           dev    #6   +/-   ##
=================================
  Coverage    0%    0%           
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  Files        2     2           
  Lines      297   278   -19     
=================================
+ Misses     297   278   -19

Impacted Files	Coverage Δ
inc/stats/namespace.php	0% <0%> (ø)
inc/helpers/namespace.php	0% <0%> (ø)

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