Nokogiri produces inefficient XPath expressions when given CSS
expressions such as "a.gfm". Luckily these expressions can be optimized
quite easily while still achieving the same results.

In the two cases where this optimization is applied the run time has
been reduced from around 170 ms to around 15 ms.

Around 300 ms (in total) would be spent in these delegated methods due
to the extra stuff ActiveSupport adds to the compiled methods. Because
these delegations are so simple we can just manually define the methods,
saving around 275 milliseconds.

This is a tad faster than letting Nokogiri figure out whether it should
evaluate the query as CSS or XPath and then actually evaluating it.

See merge request !2262

See merge request !2252

While it's useful to keep track of the different versions (Ruby, GitLab,
etc) doing so for every point wastes disk space and possibly also RAM
(which InfluxDB is all to eager to gobble up). If we want to see the
performance differences between different GitLab versions simply looking
at the performance since the last release date should suffice.

Swap Author and Assignee Selectors on issuable index view

Closes #4039

See merge request !2256

[ci skip]

Why did this break? `¯\_(ツ)_/¯`