Include isolate as a git submodule

Make the subdirectory isolate/ become a git submodule for the repository
cms-dev/isolate. We use the https:// address (instead of the git:// one)
to avoid authentication and permission issues. Also explain how to clone
in the README.

Fixes #1.
Fix compilation with isolate submodule.
Use `git clone --recursive` when suitable
Add displaying submission limits in CWS
Do not show negative numbers for remaining submissions.
Allow admins to enable/disable workers from AWS
Always ignore results from disabled workers

Not doing that had the side effect of re-enabling a disabled worker
when it reported a finished action. This is unexpected wrt timeouts,
but it would be easy to trigger now that admins can disable workers.
Fix closure in switching questions page
Add new author
Introducing triggered services.

ScoringService, PrintingService, ProxyService, and EvaluationService
have all a similar pattern (getting notified of object that needs an
action, and running a sweeper to catch missing object). This commit
introduce an appropriate base class, TriggeredService, that takes care
of setting up these mechanisms.

It also make all these services use a single priority queue (similar
to what ES used), extracting it to a reusable and tested class.

More in details. The real work on the operations to perform is
delegated to "executors", who have a single method to override,
"execute". Inside an executor, there is a priority queue of operations
to perform, and the executor sequentially picks from the top an
operation to execute (or with a configuration, can also execute all
the operations currently present in the queue at the same time).

A triggered service can have one or more executors. Each time an
operation is enqueued, or a missing operation is found, the operation
is enqueued in the queues of all executors.

In practice, the priority queue is necessary only for ES, but its nice
to have also for the other services (apart Proxy that executes all the
operations at the same time, and is therefore the only client of the
batch mode of execution). Proxy is also the only service using more
than one executor, one for each external ranking.

Nonetheless the model gives some flexibility to do additional change
in case we will need them, for example: an ES that can handle more
than one contest, and with a pool of workers assigned to each, could
have n executors, each ignoring the submission for other contests.
Converting to the lazy version of logging.
Fix queue_status and workers_status RPC methods
Add to_dict to the other operations
Make more precise the description of the arguments of checker in the doc.
Reformat the description of checker in the doc.
Allow easy inclusion of sample inputs in gen/GEN
Fix and streamline GEN file loading in YamlImporter and cmsMake.

Also, implement correct dependency tracking for #COPY: statements.
Use more descriptive names for variables.