Improvements to prompt reference segment

Problem/Motivation

Proposed resolution

1. Remove text-before/text-after config, because other segment types don't have this text-before/text-after configuration, so unify/simplify and remove this configuration from prompt (config/content) segment as well. These specific forms will have less configuration and be simpler, and be the same like other segments. User will create a separate "text" segment instead to display text before or after.
2. Add link to referenced prompt in prompt preview, so easy access/review/edit referenced prompt.

Remaining tasks

User interface changes

API changes

Data model changes

1. text before and text after configuration will be removed without data migration, because it is unlikely these futures are used at this stage.