Instead of highlighting all lines when not all of them are
needed, only highlight specific lines.
The `BlobPresenter#highlight` method has been updated to
support `since` and `to` params. These params will be used to
limit the content to be highlighted.
Modify `Gitlab::Highlight` to support `since` param which will
then be used to determine the starting line number.
Return the `text` as plain string in the response instead of
including HTML tags but keep `rich_text` as is.
The fix is to modify `Blob::UnfoldPresenter#diff_files` to map
each raw diff line (limited by the range specified) to a
corresponding line in an array of highlighted lines to use as
`rich_text`.
