MR unmergeable notification

See merge request gitlab-org/gitlab-ce!18042

First, transitions between can_be_merged & cannot_be_merged are removed,
as they are currently blocked in `check_if_can_be_merged`.
`can_be_merge` always returns to `unchecked` first,
before it can transition to `cannot_be_merged` (and vice versa).

We want to avoid repeated notification triggered by repeated transition
between `cannot_be_merged` & `unchecked`.

So we added `cannot_be_merged_recheck` state, similar to `unchecked`,
but as a mean to remember it’s from cannot_be_merged.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18042/#note_65945407

Since `unchecked` and `cannot_be_merged_recheck` both mean
“we are in the middle of checking if it is mergeable”,
quite often we need to see if merge_status is in either one of them,
so `check_state?` is added to achieve this.

Prior to this change, Flowdock used Grit to get the difference between
commits and post that to the remote service. This required direct path
access, which doesn't work with Gitaly.

Fixes gitlab-org/gitaly#1113

[10.6] Prevent notes on confidential issues from being sent to chat

See merge request gitlab/gitlabhq!2366
# Conflicts:
#	app/helpers/services_helper.rb

Server Side Request Forgery in Services and Web Hooks

See merge request gitlab/gitlabhq!2337

Improve JIRA event descriptions

Closes #25542

See merge request gitlab-org/gitlab-ce!17811