ENH Refactor the way selected options are returned for read only tag fields

[adunn49]

Description

When transforming a tag field to read only mode and calling the Field method on it for rendering, it was observed that all options in the tag field source were being returned from the database, and iterated to provide a full list on which the selected options are then filtered. Where there are large numbers of items in the source this was causing performance issues.

The getOptions method that is called for setting the values on the read only tag field accepts a $onlySelected parameter which, when set to true, will avoid the loading of all options, and instead just return the selected values. Calling the getOptions method in this way means that we no longer need the filtering within the ReadonlyTagField class.

Manual testing steps

These steps are included on issue #309

Issues

• ReadonlyTagField loads all option items, without lazy loading, prior to returning selected ones #309

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