Adds method for customising primary key and entry identifiers for social features

[tonytlwu]

Ref. https://github.com/Fliplet/fliplet-studio/issues/5704

To define a custom primary key:

Fliplet.Hooks.on('flListDataBeforeGetData', function (options) {
  options.config.dataPrimaryKey = 'Session ID';
});

To create a custom primary key based on various data:

Fliplet.Hooks.on('flListDataBeforeGetData', function (options) {
  options.config.dataPrimaryKey = function (data) {
    return _.get(data, 'record.data.Title') + '-' + _.get(data, 'record.data.Date');
  };
});

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Also follows #299 to adjust some agenda layout spacing.

[squallstar]

@tonytlwu why does the primary key attribute needs the data prefix? Could there be a primaryKey applying to a different entity?

[tonytlwu]

@squallstar I don't have any scenario for it. However, the config object describes the configuration for the LFD instance, which is feature rich. Not that I'd know why yet, but primary keys could possibly apply to comments, users etc.