add a variable for \00a0 unicode character for breadcrumb

Created by: gniquil

Currently, in the breadcrumb file:

    + li:before {
      content: "@{breadcrumb-separator}\00a0"; // Unicode space added since inline-block means non-collapsing white-space
      padding: 0 5px;
      color: @breadcrumb-color;
    }

Once it's transpiled to sass, then processed through a Sass engine, the slash in "\00a0" gets doubled to "\00a0" causing the breadcrumb to display incorrectly. We can easily factor this unicode out into a variable, which Sass will then properly escape it.

I will submit a PR for it. For reference, here are 2 issues downstream:

https://github.com/twbs/bootstrap-sass/pull/911

https://github.com/lifegadget/ember-cli-bootstrap-sassy/issues/22