Make code line numbers work as links

I added two functions to the file, one adds the links to the code line numbers
and the another one removes the link when the code line number isn't hovered.
The link is combined from the file id and the line number, which should be
enough to identify the row, which should be highlighted. The link is removed
from the th element, when user hovers on another row.

Testing Done:
Testing is done on my development server.

Reviewed at https://reviews.reviewboard.org/r/5576/
Jump to and highlight anchors in the diffviewer URL hash.

When people are passing around links to specific lines in the diff viewer, it
helps to be able to see what that link is referring to. This change adds some
highlighting to the given row, and some logic to add in the anchor if it's not
present, since the templates only add anchors for lines that have comments on
them.

Testing Done:
Testing done on my development server locally, works fine there

Reviewed at https://reviews.reviewboard.org/r/5706/