Improve flags section of the debug panel

[bprusinowski]

It would be great to enhance the flags section of the debug panel with:

• automatic listing of every available flag without a need to set it first,
• correct UI controls for all flags (currently we have a true / false switch to set a custom GQL endpoint, which results in application errors and a need to manually clear localStorage to fix the problem – this should be a text input),
• other potential improvements.