Correct new line numbers in diff viewer

With each new hunk, starting new line number was the same as starting old
line number. Parse diff line a bit further to get real new line number.

Fix HREFs for old line number links in both bootstrap3 and default themes
so that thay work.
This regex should really be something like `@@ -(\d+),?\d* \+(\d+)` so as to not break if the hunk only contains one line.
For example in some (admittedly very rare) cases you can get diffs like this:

```diff
--- a/file1.txt
+++ b/file1.txt
@@ -1 +1,3 @@
 hello world
+
+more stuff here
```