Improve DarkConsole "Services" and "XHProf" plugins

Summary:

• Services: Show summary panel of total service call costs and relative page weight.
• Services: Add "Analyze Query Plans" button, which issues EXPLAIN for each query and flags problems.
• XHPRof: iframe the profile.

Test Plan: Used the new query plan analysis to find missing keys causing table scans, see D627.

Reviewers: jungejason, tuomaspelkonen, aran

CC:

Differential Revision: 628