Upgrade Draftail to v1.2.0. Fix wagtail#4985

This includes:

- [v1.0.0](https://github.com/springload/draftail/releases/tag/v1.0.0) (identical to v0.17.2)
- [v1.1.0](https://github.com/springload/draftail/releases/tag/v1.1.0) (contains fix for wagtail#4985)
- [v1.2.0](https://github.com/springload/draftail/releases/tag/v1.2.0)

Here is the combined CHANGELOG for what's relevant to Wagtail:

Bug fixes
~~~~~~~~~

- wagtail#4985 – Prevent crash when filtering pasted content whose last block is to be removed (e.g. unsupported image) ([wagtail#179](springload/draftail#179)).
- Stop unnecessarily calling `onSave` in the editor’s `onBlur` ([wagtail#173](springload/draftail#173)).
- Prevent crash in `DraftUtils.getEntitySelection`, when the provided entity key isn't valid (undefined, missing) ([wagtail#168](springload/draftail#168)).
- Fix entity removal and editing not doing anything when the selection is backwards (right to left) ([wagtail#168](springload/draftail#168)).
- Prevent the editor from crashing when copy-paste filtering removes all of its content (thibaudcolas/draftjs-filters@652750f)

New features
~~~~~~~~~~~~

- Add support for Markdown shortcuts for inline styles, e.g. `**` for bold, `_` for italic, etc ([wagtail#134](springload/draftail#134), [wagtail#187](springload/draftail#187)). View the full list of [keyboard shortcuts](https://www.draftail.org/docs/keyboard-shortcuts).

New APIs
~~~~~~~~

- Add [`onFocus`](https://www.draftail.org/docs/api#managing-focus) and [`onBlur`](https://www.draftail.org/docs/api#managing-focus) props to use callbacks on those events. This can be useful for [form validation](https://www.draftail.org/docs/next/form-validation). [wagtail#170](springload/draftail#170), [wagtail#174](springload/draftail#174), thanks to [@TheSpicyMeatball](https://github.com/TheSpicyMeatball).
- Add [`plugins`](https://www.draftail.org/docs/plugins) API to support extensions of the editor using the [draft-js-plugins](https://github.com/draft-js-plugins/draft-js-plugins) architecture ([wagtail#83](springload/draftail#83), [wagtail#171](springload/draftail#171)).
- Add ability to disable or customise the editor toolbar with [`topToolbar`](https://www.draftail.org/docs/customising-toolbars).
- Add ability to add a toolbar below the editor with [`bottomToolbar`](https://www.draftail.org/docs/customising-toolbars).
- Add data reset parameter to `DraftUtils.resetBlockWithType()`.
- Enable list continuation on Enter for custom `*-list-item` blocks. All that’s required is for the block type to end with `-list-item`.

None of those API additions will be usable within Wagtail (at least without hacks) until corresponding rich text features APIs are built to allow their configuration. The last 2 additions would already be usable but they would only be useful when leveraging the other APIs.