Revert "Revert "pull out migration-related styles into separate file so that core styles can be used in new static site""

This reverts commit 789e1b8a8b446e882b7b0af0b21a253059952626.
add b3-migration css file to base
update navbar height handling in b3-migration css
Merge pull request #4962 from dimagi/dashboard-css

Properly pull out migration css