Change tagPicker input component to work with references

[p10e]

Right now, the tags multi-select field in ticket documents uses a tagPicker input component to load its tag options dynamically via the tagOption document type. It stores the user selection in an array of strings, which means tags are not directly linked to tag options, and therefore:

• When a user renames a tag option, tickets' tags arrays do not automatically update
• It is unnecessarily difficult to query tickets by tag

If we make tags an array of references and adapt tagPicker to work with references instead, the above issues are resolved. It also reduces the confusion around having a tag object and a tagOptions document, leaving only the need for one of them.

Also, this will make the tagPicker more robust in case we want to publish a version of it as a multi-select input plugin.