Display used package versions in interface

The playground currently makes a number of packages available to code the user submits ([list here](https://github.com/tomsmeding/play-haskell/blob/911830fdda7c89822f9bf9833903eb18014f74d6/play-haskell-worker/bwrap-files/mkbuildscript.sh#L27-L39), see also #11).

However, the user may want to know the versions of the packages being used; this should be made available in the UI. Probably by letting `mkbuildscript.sh` write a freeze file as well and parsing that; not sure what the UI element should look like, perhaps some "about" page/modal (suggestion by @Kleidukos)

Start with play-haskell-worker/bwrap-files/mkbuildscript.sh, especially the package setup around lines 27-39, and review issue #11 for context. Determine how a freeze file can be produced and parsed, then expose those package versions in the playground UI through an about page or modal; done means users can view the versions of the available packages.

Written by the indexing model from the issue text.