Fix behat tests for insert-a-link.feature

- Fix "I can link to a file" scenario.
- Fix "I can link to an anchor” scenario.
- Fix "I can edit a link” scenario.
- MINOR: Grammar fixes.
- Fix "I can remove a link” scenario.
Merge pull request #921 from madmatt/pulls/behat-insert-links

Fix behat tests for insert-a-link.feature