Bump gitlab-shell to 2.5.4

For #2042

See merge request !1584

Fix links to issues if instance is using relative urls.

Fix for https://github.com/gitlabhq/gitlabhq/issues/8767

See merge request !1574

Changes for 7.8 RC5

See merge request !1566

Change check to only swap static middleware when it's enabled.

Fixes problem with #1553.

cc @marin

See merge request !1565

Changes for 7.8.rc4

See merge request !1564

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