* Moving model-store into “generic” sub-directory.
Also adding a README.md to explain what belongs in “generic”
* Moving user-messages into “generic” sub-directory.
* Moving PageLoading into “generic” sub-directory.
* Moving “tabs” module into “generic” sub-directory.
* Moving InstructorToolbar and MasqueradeWidget up to instructor-toolbar.
The masquerade widget is a sub-module of instructor-toolbar.
* Co-locating celebration APIs with celebration utils.
Also adding an ADR about thunk/API naming conventions and making some other areas of the code adhere to it.
* Moving courseware data (thunks, api) into the courseware module.
Note that cousre-home/data/api still uses normalizeBlocks - this should be fixed so it’s not reaching across. Maybe we pull that particular API up top.
This PR includes a few TODOs for things I saw, as well as a tiny bit of whitespace cleanup.
