Fix note attachments XSS and access control

Replaces the reverted #1528, as proposed in https://gitlab.com/gitlab-org/omnibus-gitlab/issues/434, as discussed with @dzaporozhets and as summarized in #2032.

@marin Could you take a look at the nginx config and apply it to Omnibus once this gets merged?

See merge request !1553

Improve web hooks

* add git_http_url and git_ssh_url to project web hook
* add visibility_level to project web hook
* add documentation about project visibility_level in API

Fixes #2018, #2030

cc @sytse

See merge request !1560

Conflicts:
	app/models/user.rb

Code from master to 7-8-stable for RC2

See merge request !1544

Conflicts:
	app/models/user.rb

Updated guides for 7.8

Updated guides fro the 7.8 release

Fixes #2009

See merge request !1531

Fix duplicate authorized applications

On the profile applications page, display one row per authorized app instead of one row per access token.  See #1987.

cc @sytse

See merge request !1524

Remove top margin for issue/mr title

See merge request !1529

Fix double note submit

Closes #1989.

See merge request !1527