Refactor more integrations to use the DSL (1/2)

app/models/concerns/notification_branch_selection.rb

app/models/integrations/asana.rb

app/models/integrations/assembla.rb

app/models/integrations/base_chat_notification.rb

app/models/integrations/confluence.rb

app/models/integrations/discord.rb

app/models/integrations/emails_on_push.rb

app/models/integrations/external_wiki.rb

app/models/integrations/flowdock.rb

app/models/integrations/hangouts_chat.rb

app/models/integrations/microsoft_teams.rb

app/models/integrations/pipelines_email.rb

app/models/integrations/pivotaltracker.rb

app/models/integrations/prometheus.rb

app/models/integrations/unify_circuit.rb

app/models/integrations/webex_teams.rb

app/serializers/integrations/event_entity.rb

locale/gitlab.pot

spec/models/integrations/base_chat_notification_spec.rb

spec/models/integrations/prometheus_spec.rb