Search box improvements

[hot007]

• Change the greyed out text "Search" to "search datasets. e.g. temperature".

• Add "advanced search" link which takes you to the filter ESGF-like view.

• Consider adding small text below search box giving a couple of examples of what it's for.

We could remove it entirely but get user feedback first.

[paolap]

I've started implementing some changes to the front-page:

https://test.dmponline.cloud.edu.au