Allow unlimited edit time for proposals (as long as creation is enabled)

**Is your feature request related to a problem? Please describe.**

We have many use cases for proposals where it would be desired to allow unlimited edit time for editing the proposals, for example MetaDecidim. Currently this is quite clunky to setup because the only way to do it is to configure a long edit time for the "proposal_edit_before_minutes" configuration.

**Describe the solution you'd like**

I would like to be able to configure unlimited edit time for proposals, as long as the proposal creation is enabled.

I suggest we do this in case the "proposal_edit_before_minutes" configuration is set to 0 (zero).

**Describe alternatives you've considered**

We are not aware of any use cases where zero minutes edit time would be necessary. We might be wrong.

If this is the case, we should separate the "allow unlimited edit time" to a separate setting.

**Additional context**

N/A

**Does this issue could impact on users private data?**

No.

**Funded by**

Depending on the development approach, we could do this ourselves.