Rename SlackNotificationService back to SlackService

What does this MR do?

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8191#note_20310845

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25843

/cc @yorickpeterse @stanhu @smcgivern @felipe_artur @ayufan

enabled an automatic merge when the build for 2312262a succeeds

Can we at least in header describe the purpose of this classes that they are for notifications?

I did prefer the SlackNotificationsService, but what to do now :) Maybe we should have this naming problem to be solved with 9.0? What do you think @dbalexandre @yorickpeterse?

Up to this point we should also have a migration that would remove this table bloat, so the future migration should be fairly fast.

merged

mentioned in commit 191a8d7b

mentioned in commit 8e274f46

Picked into 8-15-stable, will go into 8.15.0-rc3.

removed ~149424 label