Webstart popup windows 11656

Fixes webstart security popup in main webclient window by linking to /webstart/ homepage instead: see https://trac.openmicroscopy.org.uk/ome/ticket/11656

To test:
 - On Windows, Firefox open /webclient/ - see that Java plugin doesn't appear in toolbar.
 - Click webstart button in toolbar - should open a popup window with /webstart/ - Check that webstart works OK.
 - Note that /webstart/ page doesn't have webclient and webstart displayed equally - this was confusing.
 - The webclient link will close the webstart window (if it has been opened from webclient).
 - If you open a new Tab and go directly to /webstart/ the webclient link and text is slightly different - will open webclient in same window.