Set custom targets for Import endpoints

We need to set custom targets for endpoints owned by ~"group::import" as requested in gitlab-com/gl-infra/scalability#1343. Details about the process can be found in https://docs.gitlab.com/ee/development/application_slis/rails_request_apdex.html

Web/API Endpoints identified that require custom targets

Endpoint	Type	New target duration
POST /groups/:id/export	API	X seconds
GET /groups/:id/export/download	API	X seconds
POST /import/github	API	X seconds
POST /import/bitbucket_server	API	X seconds
POST /bulk_imports	API	X seconds
GET /bulk_imports	API	X seconds
GET /bulk_imports/entities	API	X seconds
GET /bulk_imports/:id	API	X seconds
GET /bulk_imports/:id/entities	API	X seconds
GET /bulk_imports/:id/entities/:entity_id	API	X seconds
POST /projects/:id/export	API	X seconds
GET /projects/:id/export	API	X seconds
GET /projects/:id/export/download	API	X seconds
POST /projects/import	API	X seconds
POST /projects/remote-import	API	X seconds
POST /projects/remote-import-s3	API	X seconds
GET /projects/:id/import	API	X seconds
Import::AvailableNamespacesController#index.json	Web	X seconds
Import::BaseController#status.json	Web	X seconds
Import::BaseController#realtime_changes.json	Web	X seconds
Import::BitbucketController#create.json	Web	X seconds
Import::BitbucketServerController#create.json	Web	X seconds
Import::BulkImportsController#status.json	Web	X seconds
Import::BulkImportsController#create.json	Web	X seconds
Import::BulkImportsController#realtime_changes.json	Web	X seconds
Import::FogbugzController#create.json	Web	X seconds
Import::GithubController#create.json	Web	X seconds
Import::GithubController#realtime_changes.json	Web	X seconds
Import::GitlabController#create.json	Web	X seconds
Import::ManifestController#create.json	Web	X seconds
Import::UrlController#validate.json	Web	X seconds