Top links 12326

Improves configuration options for web top_links.

To test:]
 - Check the top_links in the webclient top head are unchanged, tooltips working etc.
 - Add a new link, with options:
```
$ bin/omero config append omero.web.ui.top_links '["OME", "http://www.openmicroscopy.org", {"title":"Open OME in new tab", "target":"new"}]'
```
 - After web restart, this 'OME' link with tooltip should open link in new tab.
 - List links:
```
$ bin/omero config get omero.web.ui.top_links
```
 - Choose one of these links to remove:
```
$ bin/omero config remove omero.web.ui.top_links '["History", "history", {"title": "History"}]'
```
 - Restart web and check that History link has been removed.
 - Set only specified links:
```
$ bin/omero config set omero.web.ui.top_links '["History", "history", {"title": "History"}]'
```
 - Restart web and check that ONLY History link is there.
 - Check that previous behaviour is supported (add link with no options)
```
$ bin/omero config append omero.web.ui.top_links '["Webtest", "webtest_index"]'
```