Summary:
Made UI fixes to Network Plugin (mostly to Route screens) to continue migration to the new design framework. This consisted mostly of replacing FlexColumn and FlexRow with Layout.Container and Layout.Horizontal.
Also, contains some cosmetic changes to "Mock Network Response" page.
Here is the screenshot with UI changes:

This was the old screen for comparison:

## Changelog
Network Plugin - UI changes to continue migration to Sandy design framework
Pull Request resolved: https://github.com/facebook/flipper/pull/1864
Test Plan: Manual testing to ensure that all data still displayed with new UI changes (especially the Data and Headers info in the "Route Info" section)
Reviewed By: passy
Differential Revision: D26125656
Pulled By: mweststrate
fbshipit-source-id: a25104274ed25788e5c0738ac0a9609f2cead751