Enhance consolidated config.yaml and remove extensions.yaml

[nimec01]

This PR moves the declaration for extra extensions from the extensions.yaml file to the config.yaml file.

It also adds the compile option to adjust how many compiles (default: 4) / error checks (default: 2) can run simultaneously.

The new config looks like this:

preview:
  enabledByDefault: false
  defaultHoleValues:
    "Double -> Picture": "const blank"
    "Double": "0"
    "Color": "black"
    "Picture": "blank"
    "TextStyle": "Plain"
    "Font": "Monospace"
    "Text": "mempty"
    "Data.Text.Internal.Text": "mempty"
    "Point": "(0,0)"
    "Vector": "(0,0)"
extraExtensions:
  codeworld: []
  haskell:
    - LambdaCase
    - NoTemplateHaskell
    - TupleSections

[jvoigtlaender]

For the purpose of discoverability, I'd suggest to actually add entries (with the current default values) for the compile options to the config file. (So that a user looking at that file will know there is something they can adjust.)

[jvoigtlaender]

Do we have any way of knowing/experimenting what values of maxSimultaneousCompiles and maxSimultaneousErrorChecks would "make sense" for us?

That is, are these actually limiting usage scenarios like several students in a lab room concurrently trying to render their Autotool submissions-to-be via CodeWorld?

And could our server handle larger numbers without problems?

[patritzenfeld]

Maybe we could just log when one of the semaphores is exhausted, but there's another request waiting, e.g. something like this in place of the plain MSem.with sem action calls that are currently used:

waitAndLogExhausted name sem action = do
  open <- MSem.peekAvail sem
  when (open < 1) $
    hPutStrLn stderr $ name ++ " has exhausted its available resources, but there is further demand." 
  MSem.with sem action

with waitAndLogExhausted "Compiles" (compileSem ctx) someIO

Then we can see how often requests have to queue up because of the current limit and can try increasing the values.

For how much the server can handle I'm not sure yet. I will look into what the setup is and if there's any additional limitations set on the CodeWorld instance compared to Autotool.

[jvoigtlaender]

Okay, let‘s do the logging stuff in a separate PR.

This here can be merged, I think.