Convert server "object exists" functions to async/await.

[lindapaiste]

Progress on #521

Changes:

• Refactor functions which are used by server.routes.js to use async/await syntax:
  • userExists
  • projectExists
  • projectForUserExists
  • collectionForUserExists
• Rewrite the routes to await the exists check.
• Create a placeholder sendHtml helper function for easier merging with Generate the 404 page using React on the frontend. #2168

@raclim this will definitely have conflicts with #2168. You can merge them in whatever order and I will fix the conflicts. Basically we want the sendHtml function from the 404 PR and the server.routes.js changes from this PR.

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

[PiyushChandra17]

Nicely Done