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.