This MR introduces a second way of calculating the base price of a resource using "Base Price Quantity Step".

Currently, for a resource, we could define on a Supply Line "Quantity Steps",  and set a price for each of these steps. Then, depending on the quantity of the resource, the price of the unique matching step would apply.

For exemple : 

| Quantity Step Range | Price for the step|
|------------|---------|
| 0 -> 10 | 10€ |
| 11 -> 20 | 9€ |
| 21 -> inf | 8€ |

With the current method, for an Order of 15 products, the 2 range "11 -> 20" would apply, the unit price for the products would be 9€, and the total price would be 15*9 = 135€.

The new method of calculating would apply the _Price for the step_ __to, and only to, all items of this step__. Which means, for an order of 15 products, the 10 first products would have a unit price of 10€, and the 5 next products would have a unit price of 9€, which makes in the end a total price of 145€, and a base price of 9.66€.

/reviewed-on nexedi/erp5!896

and add necessary values for calculating price per slice

To be sure that Cells are updated correctly when the pricing method is chosen

Also remove explanation_only which is dead code

This reverts commit 68d0c366.
There are other codepaths which call _postCopy without going through
__duplicate. For example: manage_renameObject (called on parent by setId).

@nexedi 
This will add a form setting field 'action title', in order to setup the text content of the form action button.
So far the action button text content is the title of the form which can be inconvenient some times. 
If 'action title' is not set-up it fallback to the form title.

![Screenshot_2019-09-04_at_18.50.04](/uploads/e3b49369f347c64a14fa0218c5923c9f/Screenshot_2019-09-04_at_18.50.04.png)

/reviewed-on nexedi/erp5!925

In testFormDialogActionTitle, check if it's well fallback to form title

See nexedi/erp5@28774376

By importing the new TimerService unconditionally,
the commit 34c81caf
(Timeout: introduce publisher_timeout configuration)
broke automatic site creation & publisher timeout
when the zope config specifies the old timerserver.

It makes to no sense to synchronize with the clock if there's nothing
after to remain synchronized.

[erp5_ingestion] Do not use absolute URL parameter
This is not compatible with ERP5JS

This is a fix for 5eeb52d5. Waitress
also requires a cancel method. Not having one raises an uncaught
AttributeError.

otherwise min / max criterion does not work.