Add condition_tag_name table to store all condition tag names for skips

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1. Add condition_tag_name table with id, unique varchar name
2. Change condition_tag.name to condition_tag.condition_tag_name_id
3. Change skip_condition_tag.condition_tag_name to skip_condition_tag.condition_tag_name_id

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An advantage of this is that, when doing the autocomplete for skips, you can easily return a complete list of previously used condition tag names. Think about how this all needs to be restricted by study in multi-tenancy scenarios.