Use `contents.downloadURL` for "save as", add feedback

[RangerMauve]

There's an API called content.downloadURL() for downloading files from a URL.

Currently we have a hacky method for Save As which executes some JavaScript at the top level of the frame instead: https://github.com/AgregoreWeb/agregore-browser/blob/master/app/context-menus.js#L205

We should replace this with invoking download() on the link that's being saved.

As well, it'd be good to have some sort of progress indicator in the web page saying that we're currently downloading something. For some reason the events for downloading are set on the "session". So maybe we should have a custom top level page which uses that for styling? I dunno, feedback would be appreciated and we could split this up into two phases.

[resession]

i'm a huge user of "save as", i love backing up stuff. this would be great.

[av8ta]

"save as replayweb archive" would be great too.