Easier way to identify what just deployed

[sven-urbanski-freiheit-com]

In release dialog:

• make it way more obvious that you're looking at the same release (e.g. by background color).
• Also disable deploy buttons if this version is already deployed.
• Display what was previously deployed. Ideally show the whole history of deployments on a dedicated page. E.g. on the release history page, show when each release was deployed. (check if this is redundant with the commit history, which wasn't rolled out yet at this point)

Suggested by @matthiasBender

[sven-urbanski-freiheit-com]

The first two points are pretty much done.