Allow default values by channels

[etienne-monsieurbiz]

Features

• Add a new node default_values_for_channels in the settings yaml to define default_values in a channel scope.
• Display the default value per channel in admin if none is saved to DB
• Update getCurrentValue() to take default channel values into account

Admin:

Front:

No values in DB:

[jacquesbh]

I have a small logic issue as well.

When I select "Use default value" again I get the default value for the Channel, but nowhere I know about it. The default value in the default form should be used since I have the data in the database.

For me, having a default value which is nowhere indicated is kind of confusing.