Truncation warning in source view is not easily visible

Issue Summary

When a file is truncated in source view, the truncation warning is at the bottom of the file... where it's not visible. One issue this causes: users don't know that browser search won't actually be looking at the whole file.

When a file is truncated in a pull request, the truncation warning is shown at the top of the file. Bitbucket's source view should do the same.

Steps to Reproduce

1. Look at a file with more than 20k lines in source view (hint: yarn.lock and package-lock.json files are often excellent candidates)

Expected Results

Like in a PR diff, a warning should be visible at the top of the file that tells the user the whole file was too large to display.

Actual Results

There is no visible warning. The warning is on the page but down at the bottom (after 20k lines). The user must scroll to the bottom of the file to discover that the content was truncated. This differs from how PR diff handles truncated files.

Workaround

Scroll to the bottom of every file you examine in source view...