Implement 'single file mode' for Large PRs

When Bitbucket's PR experience attempts to load PRs, it currently retrieves all files from a PR, and renders them simultaneously on the page. This occasionally overloads the front-end and prevents users from viewing their PRs. 

We're going to create a backup mode that loads a single file at a time, just like Bitbucket Server currently does. This will involve adding a new search functionality. 

This mode will be triggered for PRs over a certain size automatically, and might possibly be manually triggered as a default setting for users (tbd).

related ticket - https://jira.atlassian.com/browse/BCLOUD-19952