Include IconHelper in GlyphiconHelper

Needed since 393c9c2 changes glyphicon to use the `icon` helper.

Also remove duplicate `glyphicon` method declared in IconHelper.
Extract FileFieldHelper from FieldHelper

Since 9f66c9c, `file_field` does not behave as the remaining fields as
much: does not have a `form-control` class, and even adding a placeholder
does not make sense.

For better legibility, `file_field` is now extracted into its own helper.
Merge branch 'file-field'