Start checks against local sites list for allowed reverse proxy forwards

First workflow for allow reverse proxy with different site names
for ispconfig panel.

TODO: Allow only for sites known to the system itself instead
of all or none.

Add descriptive comments to table definition

See merge request ispconfig/ispconfig3!862

Add incron_plugin, fixes #5218

See merge request ispconfig/ispconfig3!864

If the plugin runs at the end, and you are deleting a Site, the deletion
will not run completely because the trigger file still exists

- reduced duplicate code in dns remoting function

Add support for powerdns 4.x, fixes #5223

See merge request ispconfig/ispconfig3!865

Add filename in the error to see what is being opened

See merge request ispconfig/ispconfig3!861

Make the debug log more specific

See merge request ispconfig/ispconfig3!847

Add README to indicate where to override these templates

See merge request ispconfig/ispconfig3!848

Limit rounding quota to 1 decimal

See merge request ispconfig/ispconfig3!846

104.3 MB looks better to me then the current 104.3345 MB. (is that level of detail is desired then maybe we should force kB notation.)

Maybe 2 or even three decimals ... but not 4.

Add domains domain update API function

See merge request ispconfig/ispconfig3!845

#5189 Warn if the lockfile for system cron is older than a day

See merge request ispconfig/ispconfig3!843

We want to avoid deadlock situations and right now if there is a process
running with the same PID that is not our process (e.g. after a reboot)
we might be waiting forever to run the cron again. So to make this
transparent to the user, we at least post a warning into the logfile.

DNSSEC support for PowerDNS

See merge request ispconfig/ispconfig3!839