Toward LLK/scratch-www#2296

This allows us to take you to any destination when clicking the logo, or to do something else, or nothing at all.

If no handler is provided, the logo will not be clickable.

The use of an "override" prop that allowed the preview modal to be hidden without actually changing the redux state was causing a problem where the block editors where not closing when modals were opening. That is because it relied on the modal reducer being accurate.

I defaulted the preview state to false to make it simpler to consume the GUI, as www will not want the preview info to be true. I made it true in the playground instead.

This fixes https://github.com/LLK/scratch-gui/issues/3710

load new project, both json and title; set title on file load; draft of code to fetch title from api

styling for title field; correct input target in tests; make download filename safer with default, max length; in playground index, correctly pass props through to gui container

If the VM is ever instantiated, which happens whenever GUI is instantiated due to default props, then IE11 will crash

/ht @gnarf

Add a meta tag to make sure google doesn’t try to autotranslate:
https://support.google.com/webmasters/answer/79812?hl=en

The initial commit for this was before we hooked up the community to redux.

And implement toggle on player playground

Add `?locale=<localecode>` to load in a new langage.
Add `?enable=language` to enable the language selector
Or combine the two: `?enable=language&locale=<localecode>`
Still accepts a hash, but requires that the hash be a sequence of digits.

Added new locales reducer to gui:
- initializes default  react-intl/localeData
- initializes default locale messages for all locales
- default locale is ‘en’ (can be overridden by URL)

Removed dependency on react-intl-redux, added ConnectedIntlProvider (uses correct locale messages from redux based on current locale)

There will probably need to be more backpack config options in the future, so just make this a bit more future proof

Since compatibility-testing got switched to use the player mode, it doesn’t need any of the stage/controls components.

* Export initial states for each reducer
* combine all gui reducers (excludes scratchPaint and intl)
* combine all initial states to create default guiState for preloading in www
* expect all of the gui state to be accessible at `state.scratchGui`

GUI will export a reducer to be included in the enclosing component’s redux when GUI is used as a library. The app-state-hoc is moved into the playground examples to provide a store when GUI is being used stand-alone.

The project loader that is exported with GUI just expects to be passed projectId as a prop, and it manages loading project data.

The part of the project loader that watches for URL hash changes for the playground is a new HOC that is only used in the various playground examples.