Show "go to latest version" on crate details pages

[Nemo157]

From #1089:

There is one remaining issue that I don't want to fix here; the "go to latest version" link doesn't appear on the non-rustdoc pages. The metadata used for rendering builds and source files doesn't currently pull enough details from the database for this, I think it might make the most sense to make both of them get the full CrateDetails and move the is_latest_version etc. checking into Tera helpers rather than being extra context variables, but that seems better done as a separate refactoring.

I think "go to latest version" on the crate details pages should link to the latest version's crate details page, rather than into the documentation (not sure whether linking directly to source/builds is worth it). It should be possible to simplify the "go to latest version" link on the doc pages as well to use the target-redirect rather than pre-calculating the path to go to, which should make it more consistent to implement across the different pages.