Added Sherlock, a custom profiling tool for GitLab

Sherlock will be a new GitLab specific tool for measuring the
performance of Rails requests (and SideKiq jobs at some point). Some of
the things that are currently tracked:

* SQL queries along with their timings, backtraces and query plans
  (using "EXPLAIN ANALYZE" for PostgreSQL and regular "EXPLAIN" for
  MySQL)
* Timings of application files (including views) on a per line basis
* Some meta data such as the request method, path, total duration, etc

More tracking (e.g. Rugged or gitlab-shell timings) might be added in
the future.

Sherlock will replace any existing tools we have used so far (e.g.
active_record_query_trace and rack-mini-profiler), hence the
corresponding Gems have been removed from the Gemfile.

Sherlock can be enabled by starting Rails as following:

    ENABLE_SHERLOCK=1 bundle exec rails s

Recorded transactions can be found at `/sherlock/transactions`.

1 files changed, 33 insertions, 0 deletions

diff --git a/app/assets/stylesheets/pages/sherlock.scss b/app/assets/stylesheets/pages/sherlock.scss
new file mode 100644
index 00000000000..92d84d9640f
--- /dev/null
+++ b/app/assets/stylesheets/pages/sherlock.scss
@@ -0,0 +1,33 @@
+table .sherlock-code {
+  max-width: 700px;
+}
+
+.sherlock-code {
+  pre {
+    word-wrap: normal;
+  }
+
+  pre code {
+    white-space: pre;
+  }
+}
+
+.sherlock-line-samples-table {
+  margin-bottom: 0px !important;
+
+  thead tr th,
+  tbody tr td {
+    font-size: 13px !important;
+    text-align: right;
+    padding: 0px 10px !important;
+  }
+}
+
+.sherlock-file-sample pre {
+  padding-top: 28px !important;
+}
+
+.sherlock-line-samples-table .slow {
+  color: $red-light;
+  font-weight: bold;
+}