store & show build logs for non-default targets

[syphar]

fixes #787.

Generally the idea is:

• we store additional logs next to the default-target logs
• the build-details view just shows all files in that folder

Due to lack of web frontend skills I just added a barely functional way to switch through the logs, I would appreciate help here, though I would also be fine with merging the interface as-is.

[syphar]

triggered by the discussion in #2424 I was looking into what's actually missing to solve this issue and more specifically, if it depends on #1011.

In the end, it was easier than I thougth.

[syphar]

Interface in this PR would look like:

[syphar]

@Nemo157 btw, I think this could also be improved by showing a build-status per target, but I definitely wanted to postpone until after #1011

[Nemo157]

I was wondering if we could just use "is the filename in the releases.doc_targets list" as a proxy for that till we actually track it. But either way better to do in a separate PR.