Fix user identities API

Added ability to update or set the identity of an existing user, like the documentation said it was possible, but actually wasn't.

@DouweM or @stanhu could you please take a look at my code, and see if its sound?

/cc @JobV 

See merge request !1398

Added ability to update or set the identity of an existing user, like the documentation said it was possible, but actually wasn't.

Clarify confirmation text on user deletion.

See #1699

Some users thought it was ok to delete the user because they'd only use the issues.  
This clarifies a bit better what's being deleted when you delete a user.

See merge request !1382

Fix CI backup step in migration guide

[ci skip]

See merge request !1397

[ci skip]

Skip check_initd_configured_correctly on omnibus installs

This was causing the task `gitlab-rake gitlab:incoming_email:check`
to fail.

See merge request !1396

Add better deprecation notice for the upgrader. Too easy to miss otherwise https://twitter.com/0x663030623472/status/646262552796594176

This was causing the task `gitlab-rake gitlab:incoming_email:check`
to fail.

8.0.0 is released; add 8.0.1 entry to CHANGELOG

[ci skip]

See merge request !1394

[ci skip]

Update CHANGELOG

Add to migration on what to do if you don't want your data.

If you don't want to migrate your CI data , remove the old builds directory.

See merge request !1392

Mention Reply by email in installation doc.

Added the "Enable Reply by email" section and moved the others around.

See merge request !1390

Ci migration doc improvements

Use export improvements in CI 8.0.1, make upgrades a more prominent part of the process, do not shut down GitLab during CI data import.

See merge request !1387

8.0.0 has been released

documentation on ssl verification in web hooks



See merge request !1386

Fix CI permissions

![Screen_Shot_2015-09-21_at_20.23.16](https://gitlab.com/gitlab-org/gitlab-ce/uploads/6f40699507d0dce148d0054a093f0d63/Screen_Shot_2015-09-21_at_20.23.16.png)

This fixes two problems:
- Allow developers to retry builds
- Hide advanced project options from CI page for non-admin users



See merge request !1379

Add missing rake and change nginx config for CI migration

/cc @marin

See merge request !1385

Reformat and copy edit the CI-to-CE migration guide

[ci skip]

See merge request !1335

[ci skip]

Tweak remaining CSS for Markdown preview

See merge request !1383