Fixes #2098

Closes #2867

Closes internal https://dev.gitlab.org/gitlab/gitlabhq/issues/2269

Closes #1305

See #1809.

Closes #1932.

Fixes #8748.

signe-in -> signed-in
go_to_gihub_for_permissions -> go_to_github_for_permissions
descendand -> descendant
behavour -> behaviour
recepient_email -> recipient_email
generate_fingerpint -> generate_fingerprint
dependes -> depends
Cant't -> Can't
wisit -> visit
notifcation -> notification
sufficent_scope -> sufficient_scope?
levet -> level

* send a 'In-Reply-To' header along the 'References' header
* subject of answers to an existing thread begins with 'Re: '

This reverts commit 97fd990e, reversing
changes made to f451a697.

Conflicts:
	app/mailers/emails/notes.rb
	app/mailers/emails/projects.rb

* send a ‘In-Reply-To’ header along the ‘References’ header
* subject of answers to an existing thread begins with ‘Re: ’

This fixes threading with at least Mail.app and Airmail.

This fixes email threading in Mail.app, that doesn't like when a thread
doesn't have stable recipients.

For instance, here is a possible sender-recipient combinations before:

From: A
To: Me
New issue

From: B
To: Me
Reply on new issue

From: A
To: Me
Another reply

Mail.app doesn't see B as a participant to the original email thread,
and decides to break the thread: it will group all messages from A
together, and separately all messages from B.

This commit makes the thread look like this:

From: A
To: gitlab/project
Cc: Me
New issue

From: B
To: gitlab/project
Cc: Me
Reply on new issue

From: A
To: gitlab/project
Cc: Me
Another reply

Mail.app sees a common recipient, and group the thread correctly.

Added email threading for update emails on issues and merge requests (if the mail client support the References: mail header)

This changes the email "From" field from "gitlab@example.com" to either:

* "John Doe <gitlab@example.com>" if the author of the action is known,
* "GitLab <gitlab@example.com>" otherwise.

Rationale: this allow mails to appear as if they were sent by the
author. It appears in the mailbox more like a real discussion between
the sender and the receiver ("John sent: we should refactor this") and
less like a robot notifying about something.

For instance, the email "Subject" field changes from:

    Mattt / Ground Control | Saving issue doesn't work sometimes (#1234)

to:

    Ground Control | Saving issue doesn't work sometimes (#1234)

Rationale:

* Most people are receiving email notifications about a single fork:
the project activity, issues and merge requests are happening either
in a root repository or in a fork, but less often in several forks.

* It removes noise from the email subject. The namespace of the project
can still be read in the email body.

* For Entreprise users that have a single namespace "MyEntreprise",
having this repeated in every email subject is very noisy.