backend for new url structure.

WIP: Dashboard & Persistent urls - Frontend Step

* origin/7883_frontend_step:
  dashboards: make scripted dashboards work using the old legacy urls
  dashboards: redirect from old url used to load dashboard to new url
  dashboards: add new default frontend route for rendering a dashboard panel
  dashboards: fix links to recently viewed and starred dashboards
  dashboards: use new *url* prop from dashboard search for linking to dashboards
  dashboards: when saving dashboard redirect if url changes
  dashboards: add new default frontend route for loading a dashboard
  dashboards: return url in response to save dashboard. #7883

Scripted dashboards are still requested from /dashboard/script/scripted.js
#7883

If legacy backend routes (/dashboard/db/<slug> and /dashboard-solo/db/<slug>)
are requested we try to redirect to new routes with a 301 Moved Permanently
 #7883

New default route /d-solo/<uid>/<slug of dashboard title> where dashboard
panel are rendered by unique identifier and panel identifier.
If old route /dashboard-solo/db/<slug of dashboard tile> are used,
try to lookup dashboard by slug and redirect to new default route.
Change url logic for sharing a panel so that the new default route
for rendering a dashboard panel are used.
 #7883

Use new property url from dashboard search for linking to dashboards
#7883

#7883

Redirect if new dashboard created or existing url changes.
#7883

New default route /d/<uid>/<slug of dashboard title> where dashboard
are loaded by unique identifier.
If old route /dashboard/db/<slug of dashboard tile> are used,
try to lookup dashboard by slug and redirect to new default route.
 #7883

Without this the uid will not be sent to the backend when saving a
dashboard.
#7883

Since we're already have possibility to get a dashboard by slug
it makes little sense to have a separate endpoint in api for
retrieving uid by slug.
#7883

Use shortid.MustGenerate() instead of shortid.Generate(). Instead
of returning errors it will panic.

#7883

Introduces new url in api /dashboards/<uid> for fetching dashboard by unique id
Leave the old dashboard by slug url /dashboards/db/<slug> for backward
compatibility and for supporting fallback
WIP for #7883

Make the max length of uid longer in case we want to change it later
#7883