Server only presents GHC versions that all workers support

[Currently](https://github.com/tomsmeding/play-haskell/blob/efc0d575166b741a7ea73b0c1187c730ab0d471b/play-haskell-server/src/Play/WorkerPool.hs#L552), the server computes the list of available GHC versions to be presented to the user as the intersection of the availability sets of all workers currently registered on the server. This is _sound_, but rather pessimistic if there are multiple workers with unequal sets.

However, it is unclear whether this really needs addressing immediately: if different workers have different available version sets, and a some popular version is available only on a fraction of the workers, then those workers would (in the fixed system) get a disproportionate number of jobs. Hence you probably want to ensure your workers have consistent version sets anyway.

Start at play-haskell-server/src/Play/WorkerPool.hs around line 552, where the server computes the presented GHC version list from registered workers' availability sets. Read the surrounding worker registration and job-selection logic, then determine the desired behavior for unequal worker sets. Done requires an agreed selection policy and corresponding tests, but the issue does not specify that policy.

Written by the indexing model from the issue text.