Convert all fixed strings to use FormattedMessage (or similar)

[paulkaplan]

All hard-coded strings (i.e. "blocks", "costumes", "sounds" in the tabs) should use FormattedMessage or similar from the react-intl package.

A few things that are important:

• You have to use either FormattedMessage or defineMessages in for the babel plugin to automatically pick them up.
• You must include all three properties to those methods: (1) english text, (2) a globally unique id in the style of the other ids, (3) a "description" which can act as a translator hint about what the contextual meaning of the string is.
• The naming convention should be gui.<component|container>. - this means existing messages need to be updated as we didn't have a naming convention when we started.

---

To aid in finding those places, you can add rules: {'react/jsx-no-literals': [2]}, to the eslintrc, which will error linting on all literals in jsx.

[chrisgarrity]

Fixed in #1920