Allow table widget to have a minimum and maximum of items

[bennothommo]

This PR adds the config variables minItems and maxItems for the Table widget and datatable field type. The impetus of this is to allow a table to have no items by setting the minItems to 0 - I have found in my travels that if you have any sort of validation on the table widget, then you must have at least one record, but in my case, I wanted the datatable field to be optional.

I then thought it might be useful to have the opposite, and allow an upper limit for the number of records.

Some rules on the above:

• minItems must be 0 or above. By default, it is 1 to match the current functionality. If minItems is 2, then 2 records must be in the datatable at all times (the datatable will have 2 empty records added on load).
• maxItems must be either 1 or above, or null to indicate no limit.
• minItems must be less than or equal to maxItems, to allow a datatable to have a fixed number of records.

To allow this change to happen, I've decided to change the datasource objects (client and server-side data handling) to use promises - this way, we have a common way of reading the responses, and it will gracefully handle the server-side taking a little longer to make changes to the data.

[LukeTowers]

@bennothommo what's the status of this?

[bennothommo]

I'm gonna say I think it is complete... Definitely haven't tested it quite some time though.

[LukeTowers]

@bennothommo do we have a docs PR for it?

[bennothommo]

I will test it first, and then I'll put up a docs PR, in case anything needs to change.

[LukeTowers]

@bennothommo what's the status on this?

[bennothommo]

Same as before. Still needs docs and testing. Haven't had a need for it yet - I think I did this for my old job.

[LukeTowers]

@der-On or @WebVPF, either of you bored and looking for something to do 😉?

[der-On]

@LukeTowers Bored to the bones 😹 . Sorry I'm currently out on this.