Add initial saved views widget implementation

[roluk]

Portions of this code are in various state of completion.

• @itwin/saved-views-client is closest to being done.
• <SavedViewTile /> consumer API is pretty much finalised, however, interactions with hover menus are clunky. I expect to improve this after upgrading to iTwinUI 4.
• I came up with the solution for how to do string localization but it is only used with <SavedViewTile /> so far.
• Build process for the @itwin/saved-views-react package incorporates a step which modifies CSS class names that start with svr- to begin with itwin-svr0__ to prevent accidental name collisions.
• The test app has been partitioned into /itwinjs and /components routes. Any stored links to the test app are now broken.
• The test app allows to browse Saved Views of published iModels but opening the view is not implemented. The app allows to make read and write Saved View requests to iTwin Platform but the result is only visible after manually refreshing the page. Expect bugs because this is the least developed part of the PR.