Sstudent/upgrade xblock

Add an OEP-2 compliant openedx.yaml file

Fix bug: Single-instance buttons do nothing after disabled

This is to fix open-craft/problem-builder#118, where a button marked with
'single-instance' data got disabled, hence its click event handler at
edx-platform is not called.

This patch solves this issue by correcting the calling order, so that we
invoke the click event handler at edx-platform first; then we disable
the button.

Fix translation of field names and help text in studio_view

Update xblock-utils version in setup.py

Support loading resources that contain non-ASCII characters

Sprint Zug PHX-201 Allow HTML to be included in XBlock Field help strings

Add helper for rendering Mako templates

Fix minor bug with preview_view feature.

Allow blocks to nest utils-unaware blocks.

Fix: Make sure EDIT dialog for block fields always operates on up-to-date data

To achieve this, remove TinyMCE from block fields on "Save" and
"Cancel" (previous version of the code was delaying this step until user
requested an EDIT dialog for another block).

Added boilerplate parameter for Nested blocks spec + test

Helper class providing studio views for XBlocks that can have nested XBlocks

Date selection support

Update XBlock commit hash to latest version.

The tests will fail otherwise with the latest version of XBlock.

Set the base URL of TinyMCE before initialising it.

Mixins to simplify accessing SettingsService and theming

Minor improvements to test base classes

Tell TinyMCE not to change our URLs!