Cleanup: Single source of default project contents

[lindapaiste]

Fixes #1423

Makes it easier to change the default project contents (for example, bumping the p5 version) because it doesn't need to be changed in so many different places.

Changes:

• Choose server/domain-objects/createDefaultFiles.js as the "single source of truth" for default file contents and export the local constants defaultCSS, defaultHTML, defaultSketch.
• Other files which duplicate these constants now import them createDefaultFiles.js instead. (server/scripts/examples.js, client/modules/IDE/reducers/files.js)
• Export the initialState function from client/modules/IDE/reducers/files.js and use this function instead of duplicating the default files array. (client/modules/Preview/, client/testData/testReduxStore.js)

Next Steps:

• There are still some files on the server with very repetitive code for creating files arrays. These can be cleaned up in a future PR by creating a utility function.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[raclim]

This is super helpful, thanks so much for working on this! Going to add this in too :)