diff: drop per file ignore-whitespace and context - it didn't work and had conceptual issue (Issue #344)

Diffs are currently generated at the low level as one big diff between two vcs
resisions, provided global values for diff context size and flag for ignoring
whitespace. All files use the same flags. There is no way to actually compute
the full diff using these use per file flags, and no simple and efficient way
to add it.

The best option is thus to drop the failed attempt at making it per file, and
just rely on the simple global flags in the URL.

The links for changing whitespace and context is sometimes shown for the whole
"page", and sometimes next to the diff for one file. For now, keep showing the
link in these places, but make sure it navigates back to the FID of the section
where the link was clicked.

The implementation is completely rewritten and moved to a more appropriate
location in helpers.

With a more clean implementation, we also consistently use the simple getters
to extract values from the URL.