Fix spec failures and add a feature flag for the performance bar

Signed-off-by: Rémy Coutable <remy@rymai.me>

lib/gitlab/performance_bar.rb

 module Gitlab
   module PerformanceBar
     def self.enabled?
-      ENV["PERFORMANCE_BAR"] == '1'
+      Feature.enabled?('gitlab_performance_bar')
     end
   end
 end

spec/models/commit_spec.rb

@@ -21,7 +21,7 @@ describe Commit, models: true do
 
     it 'caches the author' do
       user = create(:user, email: commit.author_email)
-      expect(RequestStore).to receive(:active?).twice.and_return(true)
+      expect(RequestStore).to receive(:active?).and_return(true)
       expect_any_instance_of(Commit).to receive(:find_author_by_any_email).and_call_original
 
       expect(commit.author).to eq(user)

vendor/assets/stylesheets/peek.scss

@@ -6,8 +6,13 @@ header.navbar-gitlab.with-peek {
 }
 
 #peek {
-  background: $black;
+  position: fixed;
+  top: 0;
+  left: 0;
+  width: 100%;
   height: 35px;
+  z-index: 400;
+  background: #000;
   line-height: 35px;
   color: #999;
 
@@ -53,12 +58,12 @@ header.navbar-gitlab.with-peek {
   }
 
   strong {
-    color: $white-light;
+    color: #fff;
   }
 
   table {
     strong {
-      color: $black;
+      color: #000;
     }
   }
 
@@ -90,5 +95,5 @@ header.navbar-gitlab.with-peek {
 }
 
 #modal-peek-pg-queries-content {
-  color: $black;
+  color: #000;
 }