removed extra_sign_in_text examples for gitlab.rb [ci skip]

I removed the extra_sign_in_text option documentation. This is no longer needed because a welcome message aka. extra_sign_in_text can now be set/changed in the admin UI.  

Admin area > Settings

This was mentioned in issue #3006

See merge request !1565

LDAP email address downcasing

Fixes #2960

In the event we cannot match an LDAP user by DN we attempt to find an identity by email address and then update the DN. In this case the identity is matched by email address. 

If the user's email address in LDAP has an upper case character we cannot find a match in the GitLab database. GitLab downcases emails before the user object is saved.

This merge request downcases the email from LDAP before we lookup by email. I also added a test to prevent a regression.

See merge request !1550

Remove slim

Closes #2897 

See merge request !1537

[ci skip]

Persist filters when sorting on user admin page

Closes #2806

See merge request !1543

In SSH key detailed view, align the "Remove" button with the right container

Also fixes a bootlint warning.

Before:

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/d6d8765ed9dceb6a31a6b4475b2166e1/image.png)

After:

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/8252211125ace212335aa6bf2ba78549/image.png)


See merge request !1555

Make diff file view easier to use on mobile screens

Viewing diffs on a mobile screen is a bit of an awkward experience at the moment. Here are a few issues (by no means complete):

## Before

### Tabs are scrunched

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/45924e36cfb107dc56795a7dcdfbc5b5/image.png)

### Filenames take too much room, buttons cluttered

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/a5e5611271fe26dfd3922aa0b8780458/image.png)

## After

This MR makes a few tweaks to make this a bit better. It just addresses a few issues, but there is plenty of room for improvement (e.g. shrink fonts, etc.):

### Eliminate padding to make tabs fit

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/fbcf128d1f221b9b9e53df274238dc62/image.png)

### Make filenames, buttons more readable

This screenshot allows the filename to use the whole row, omits the file mode changes, and puts the buttons centered in the view:

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/d2e5b9424f058827ba2552056c516e18/image.png)

Towards a better mobile experience: #2787 

See merge request !1449