DS-1995: JSPUI Bootstrap dropdowns don't display properly in IE8 or 9

See https://jira.duraspace.org/browse/DS-1995 for details.

This PR:

1. Removes the CSS `filter` rules on the header navbar which are causing the dropdown to display incorrectly.
2. Adds Paul Irish's conditional comments hack which allows specific IE versions to be targeted.
3. Adds a `background-image` rule for the header navbar targeting IE9 only, which provides a SVG gradient similar to that provided by the vendor-prefixed `background-image` gradients. IE8 does not support SVG, so it is not targeted. With this PR, IE8 displays the header navbar with the default `background-color`.

I'm not very active in this community, and I've never submitted a pull request before, so hopefully I'm doing this right...