Webstart popup windows 11656 develop

--rebased-from #1781

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.