vcs: Return merge reference in merge response. Previously we returned only the merge commit id. Now we set a bookmark(hg) or branch reference(git) to point to the merge commit. By returning a merge reference we have both informations available (id and name)

pep8: Remove unused imports and variables.

pr-shadow: Use reference object to store the last merge reference of a PR. #1055

db: Refactor reference conversion to an reusable method.

tests: Add test to check the link to the shadow repo of a PR.

tests: Add a method to AssertResponse to check value of elements.

pr-shadow: Display link to last merge revision in shadow repository if merge was successful. #1055

pr-model: Commit changes to DB on merge status update.

pr-shadow: Set last merge revision on merge status update.

pr-shadow: Copy last merge revision if creating a PR version.