[FEATURE] Contact field and localstorage improvements

[chrismclarke]

What?
Following work in #2320, a number of follow-up improvements identified for 1 or more PRs.

Contact Field Implementation

• Add a lightweight migration system to allow us to handle future localstorage value changes
• Replace rp-contact-field prefix with app deployment name (migrate existing, decide whether to keep prefix when syncing to server or not)
• Consider refactoring localStorage service to store json objects that include data type
• Consider implementing with RXDB to store localstorage values in-memory and persist to disk asynchronously (similar to dynamic-data service)

Code Tidying

• Consider refactoring rest of appConfig fields to protected (e.g. selected_text_field, app_update_available_field)
• Consider simplifying templateFieldService to be more of a loose wrapper around localstorage service - if implemented with rxdb could expose value subscription methods
• Consider removing setJson and getJson methods
• Remove final references to rp-contact-field from test suites
• Consider refactoring the app-events.service to not persist a combined value to localstorage and instead just track a few protected field entries
• Refactor task.service to use protected fields instead of arbitrary setField operations and highlightedTaskField variable