Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Adds sorting to deployments API through the `order_by` and sort
`fields`.

[10.3] Migrate `can_push` column from `keys` to `deploy_keys_project`

See merge request gitlab/gitlabhq!2276

(cherry picked from commit f6ca52d31bac350a23938e0aebf717c767b4710c)

1f2bd3c0 Backport to 10.3

Enables project milestone deletion via DELETE /projects/:id/milestones/:milestone_id

This removes all usage of soft removals except for the "pending delete"
system implemented for projects. This in turn simplifies all the query
plans of the models that used soft removals. Since we don't really use
soft removals for anything useful there's no point in keeping it around.

This _does_ mean that hard removals of issues (which only admins can do
if I'm not mistaken) can influence the "iid" values, but that code is
broken to begin with. More on this (and how to fix it) can be found in
https://gitlab.com/gitlab-org/gitlab-ce/issues/31114.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/37447

States this service was deprecated on 10.3 and his behavior depends on
the kubernetes service status. Also includes a link for the release post
for more information.

+ Add support for the new separate channel and events settings
* Dry up chat notifications in the service properties definitions

- Apply some refactoring for code reuse
- Add file status validation for Files::DeleteService
- Write additional specs