refactor: rename isUnmergeable to isConflicted

lib/platform/bitbucket/index.js

lib/platform/github/index.js

lib/platform/gitlab/index.js

lib/platform/vsts/vsts-helper.js

lib/workers/branch/parent.js

lib/workers/pr/index.js

lib/workers/repository/finalise/prune.js

lib/workers/repository/onboarding/pr/index.js

test/platform/bitbucket/__snapshots__/index.spec.js.snap

test/platform/github/__snapshots__/index.spec.js.snap

test/platform/gitlab/__snapshots__/index.spec.js.snap

test/platform/vsts/__snapshots__/vsts-helper.spec.js.snap

test/platform/vsts/vsts-helper.spec.js

test/workers/branch/parent.spec.js

test/workers/pr/index.spec.js

test/workers/repository/finalise/prune.spec.js

test/workers/repository/onboarding/pr/index.spec.js