Enable Performance/StartWith cop and fix offenses

.rubocop.yml

@@ -985,11 +985,10 @@ Performance/RedundantMerge:
 Performance/RedundantSortBy:
   Enabled: false
-# TODO: Enable StartWith Cop.
 # Use `start_with?` instead of a regex match anchored to the beginning of a
 # string.
 Performance/StartWith:
-  Enabled: false
+  Enabled: true
 # Use `tr` instead of `gsub` when you are replacing the same number of
 # characters. Use `delete` instead of `gsub` when you are deleting

app/helpers/gitlab_markdown_helper.rb

@@ -13,7 +13,7 @@ module GitlabMarkdownHelper
   def link_to_gfm(body, url, html_options = {})
     return "" if body.blank?
-    escaped_body = if body =~ /\A\<img/
+    escaped_body = if body.start_with?('<img')
                      body
                    else
                      escape_once(body)

app/helpers/tab_helper.rb

@@ -112,7 +112,7 @@ module TabHelper
   end
   def profile_tab_class
-    if controller.controller_path =~ /\Aprofiles/
+    if controller.controller_path.start_with?('profiles')
       return 'active'
     end