Add Plugins menu in the header

Administration dropdown menu should be created and it should have 2 sub-items: Admin mode & Plugins.

• Admin mode should lead to the same changes as current Administration Mode menu does. When it's pressed, next time user sees this dropdown, Admin Mode should change to Exit Admin Mode
• Plugins menu item should lead to the newly created page with the list of plugins

Here are a mockups:

• http://jira.jtalks.org/secure/attachment/13507/mockup_plugins-menu.png
• http://jira.jtalks.org/secure/thumbnail/13508/_thumb_13508.png