This change has two aims:
1. To fix some flakiness that recently appeared
2. To restore the test of creating a file via the blob viewer

- Uses specific predicate methods to check for content instead of
  checking the entire page. This also implicitly waits for loading to
  complete, which should make the test more reliable.
- Reverts Resource::File to create a new file via the blob viewer,
  after initializing the project with a readme.
- Removes some unused code.
- Adds a page object component for the flash notice that appears on
  many pages.