Revert "disable user_line routes and tests"

This reverts commit b328f5dce046c76719fb8076c69c2aec1adb2d2f.
write initial scenarios for line <-> extension linking
move line routing into route package
add endpoint for associating a line with an extension
implement GET /line/<id>/extension
implement line extension dissociation
randomize ids in scenarios
simplify steps used to check links
add missing prerequisite in a scenario
refactor scenarios for associated a line_extension
line_extension should return links towards a line
fix grammar in error message
fix error messages in scenarios
reuse steps already implemented for prerequisites
reuse steps for checking response
fix failing unit tests
rename feature file
regroup 2 steps into a prerequisite
fix resource link
simplify prerequisite steps in user_line scenarios