Allow multiple snapshots

For testing of modules you often need different sets of activated modules. For example, if you want to test a module during development, devel.module and coder.module should be activated after a reset.

The goal of this feature request is to allow different snapshots that contain certain modules. We need to

• save the list of active modules for each dump (f.e. in a separate text file)
• allow to enter a name for a new snapshot (a prefix for default name)
• allow to select from available snapshots in the request form (a radio list that displays the modules in descriptions)
• allow to select the snapshot that is used for resets in cron runs