Add permalink, including view preferences

[tripu]

Instead of #24, have a “permalink” or “link to this page” somewhere on the page.

That URI will include the current “view settings” (thread view, formatting, extended headers, etc), either as fragments (⋯/0002.html#thread=on) or as parameters (⋯/0002.html?thread=on&headers=simple&formatting=md). @gosko seems to prefer the former, for cacheability — but we have to think how to encode more than one tuple as a fragment, and also decide which is best for crawlers, non-JS UAs, etc.

[gosko]

I don't think we need to include parameters for things like the short/detailed header toggle or nice/plain formatting, maybe just 0002.html for a single message, 0002.html?view=thread for the entire thread, and 0002.html?view=flattened for the flattened view? (and maybe we can come up with a better name than 'flattened': inline? chrono?)

[tripu]

I don't think we need to include parameters for things like the short/detailed header toggle or nice/plain formatting,

Maybe not. But I'd like to have a mechanism that can be easily extended. It's possible that we will want to encode more than one piece of configuration in the URL at some point; if we are going to include display options in the URL, we better do it in a way that is not limited.

Maybe we can come up with a better name than 'flattened': inline? chrono?

I suspect the name will present itself when that feature is implemented and we know exactly how it works :)

[gosko]

also, as you might have guessed from the inconsistencies between my previous comment and my comments in #24 I am not completely sure whether to use fragments or query strings for things like this; maybe an implementation will reveal the pros/cons of each :)