dashboard json cannot contain fixed id when importing from
disk. We used to override this but it didnt caught all problems
so now we block dashboards from beeing imported instead.

closes #10504

* backend_plugins: (34 commits)
  code style fixes
  Adds Table in backend datasource contract.
  Adds Tables types to protobuf
  Review tsdb protobuf contract
  supports windows compatible plugin binaries
  moves plugin proxy to plugin package
  improves name for plugin logger
  uses pluginmanagers log instead of global
  removes commented code
  makes datasource handshake more explicit
  backend plugins: improves logging
  dont spawn new subprocess while shutting down
  plugins: restart killed plugins
  query result should be a map
  test for plugin path builder
  merge backend datasources and datasources
  use int64 for timestamps
  fixes invalid valud/timestamp order
  merge backend-datasource and datasource type
  fixes broken unit test
  ...

* master: (117 commits)
  fix: share snapshot controller was missing ngInject comment, fixes #10511
  Use URLEncoding instead of StdEncoding to be sure state value will be corectly decoded (#10512)
  Optimize metrics and notifications docs
  Optimize cli and provisioning docs
  docs: Guide for IIS reverse proxy
  changelog: adds note about closing #9645
  telegram: Send notifications with an inline image
  telegram: Switch to using multipart form rather than JSON as a body
  telegram: Fix a typo in variable name
  fix: alert list pause/start toggle was not working properly
  fix template variable selector overlap by the panel (#10493)
  dashboard: Close/hide 'Add Panel' before saving a dashboard (#10482)
  fix: removed unused param
  Fix variables values passing when both repeat rows and panels is used (#10488)
  moved angular-mocks out of dependencies
  ux: minor change to alert list page
  ux: minor word change to alert list
  fix: updated snapshot test
  Add eu-west-3 in cloudwatch datasource default's region (#10477)
  fix: Make sure orig files are not added to git again #10289
  ...

Fix docs

Add support for inline image uploads to telegram notifier plugin

If image upload is enabled, but there is no public image repository,
fall back to sending an image via telegram's API.

This is a pre-requisite for uploading inline images.

Fixes #10472

* dashboard: repeat rows and panels tests

* fix variable values passing when both repeat rows and panels is used

* Add eu-west-3 as a possible default datasource

* Add eu-west-3 as a possible default datasource