Dash-devices

Fixed
- [320](https://github.com/plotly/dash/pull/320) Disallow duplicate component ids in the initial layout.

Fixed
- Fix bug [321](https://github.com/plotly/dash/issues/321) where importing Dash components with no props would result in an error.
- Fix a bug in 0.23.1 where importing components with arguments that are python keywords could cause an error. In particular, this fixes `dash-html-components` with Python 3.7.

Added
- [319](https://github.com/plotly/dash/pull/309) Add a modified time query string to assets included in the index in order to bust the cache.

Added
- [316](https://github.com/plotly/dash/pull/316) Add `ie-compat` meta tag to the index by default.
- [305](https://github.com/plotly/dash/pull/305) Add `external_script` and `external_css` keywords to dash `__init__`.
- Dash components are now generated at build-time and then imported rather than generated when a module is imported. This should reduce the time it takes to import Dash component libraries, and makes Dash compatible with IDEs.

Fixed
- [273](https://github.com/plotly/dash/pull/273) Raise a more informative error if a non-JSON-serializable value is returned from a callback.

Added
- [286](https://github.com/plotly/dash/pull/286) Asset files & index customization.
- [294](https://github.com/plotly/dash/pull/294) Raise an error if there is no layout present when the server is run.
- [renderer55](https://github.com/plotly/dash-renderer/pull/55) Add `_dash-error` class to the "Error loading layout" and "Error loading dependencies" messages.

Fixed
- Attempting to render a `Boolean` value to the page no longer crashes the app.
- [renderer57](https://github.com/plotly/dash-renderer/issues/57) If a callback references an `id` which does not exist in the DOM tree at the time it is executed, throw a more informative front-end exception.
- [renderer54](https://github.com/plotly/dash-renderer/pull/54) Previously, if a component called `updateProps` with multiple properties, Dash would fire the callback multiple times (once for each property). Now the callback only fires once.