This change adds function/class header information to collapsed regions in the diff viewer and to commented regions on reviews.

New functions have been added to the differ classes for registering "interesting line" regexes. These are regular expressions associated with a name that can match lines in a file. Any matched lines are stored in a list associated with that name. Multiple regular expressions can be tied with a name.

A list of regexes for various common file formats are defined that match functions/classes. After performing the diff, we grab the matched lines and store them along with the chunk. We can then use these to show the appropriate function/class in a collapsed region.

Similarly, we can grab out these headers for commented regions of diffs on the review page and in the e-mails.

Ran a suite of unit tests for each file format we support. They all passed as expected. Tested these with complex Python files and C files and saw the expected results as well.