Issues this addresses

1. when opening an item page with any sidebar content, #links is always added to the url if there is a IIIF manifest but no annotations or anno search, but this is undesirable if the sidebar is not open
2. back/forward browser navigation goes between tabs on item page before going back to e.g. search results

Changes

activeTab composable:

• don't watch activeTabIndex by default, but return a function to start watching on-demand, plus one to stop watching
• switch to using router replace so that browser back/forward navigation doesn't loop through the tabs

ItemMediaSidebar:

• add a show prop to control visibility
• only watch activeTabIndex changes when component is shown (according to that prop)
  • preventing immediate triggering of it on initialisation always selecting the first tab's index & hash

ItemMediaPresentation:

• pass show prop to ItemMediaSidebar based on value of showSidebar