Admin CSS: Make suggest script visible in customizer

[celloexpressions]

Related: #25618. We need to assign a much larger z-index for the suggest script so that it is visible in the customizer. Currently it is hidden behind everythign else (see ​http://core.trac.wordpress.org/ticket/25549#comment:31).

This is needed for Twenty Fourteen's featured content implementation.

[celloexpressions]

Bump up the z-index for the suggest script's div so that it is visible in the customizer.

[celloexpressions]

Patch gives the suggest script a high enough z-index to be visible in the customizer. Unfortunately this is much higher than the admin bar's z-index, but there's no way to distinguish between the admin and the customizer with css so we'll have to go with it. Only other option is to make the z-indicies on the customizer saner, although that could break things (wondering why they're so high in the first place).

[lancewillett]

Oops, bad title change, sorry.

[celloexpressions]

Patched against mp6's wp-admin.css.

[SergeyBiryukov]

25867.2.diff​ adds wp-customizer class to only target the customizer.

[SergeyBiryukov]

25867.3.diff​ also removes the code for adding the inline style from Twenty Fourteen.

[SergeyBiryukov]

In 26085:

Make suggest script visible in the customizer.

props celloexpressions for initial patch.
see #25867.

[SergeyBiryukov]

In 26086:

Remove the code for increasing z-index for the tag suggestion script from Twenty Fourteen.

fixes #25867.