[hooks] Do not register "logstats" if repository has no scheduler

[server] Introduce an `has_scheduler` method on Repository This is to be used by client application to determine if looping tasks may be registered in the current process. By checking this, one will avoid the warning in looping_task method when the repository has no scheduler.

[server] Warn instead of failing when a looping task is registered and repo has no scheduler We should provide a way for client code to detect if they should register the looping task or not. See next patch for that.

[server] Exit quickly when a looping task is registered in maintenance mode In such cases the repository will not have a scheduler on purpose because the repository will not be kept running and will quickly shutdown after migration so that it's undesirable to have looping tasks being executed.

[config] Drop unused init_available_cubes method

[doc] Move content of former README.pyramid into relevant modules

Drop README.pyramid.rst file It has been kept since 3.24 when we merged pyramid-cubicweb and cubicweb-pyramid repositories into cubicweb's one waiting for someone to move the information in some canonical place but this never happened and this information is probably not so useful or already existing elsewhere. So just drop the file.

Make entypo link a true URL in README

Refer to cubicweb.readthedocs.io in README docs.cubicweb.org contains outdated documentation at the moment (and for quite some time); until it gets update, refer to the up-to-date documentation on readthedocs.

[server] don't catch exception from postcommit_event() if we are in test mode The historic behavior is to hide potential exception occurring in postcommit_event(). Unfortunately logging statements are hidden by default during tests and it become very hard to debug. At least raise if we are in test mode.