Avoid hardcoded strings across the codebase

[bigspider]

There are many instances were a value can only be one of multiple strings, like the appointment status that can be "not_found", "being_watched" or "dispute_responded". In this and all the other similar cases we should avoid hard-coding the string, for example by using an Enum. For example we could have and AppointmentStatus enum, with keys AppointmentStatus.NOT_FOUND, AppointmentStatus.BEING_WATCHED and AppointmentStatus.DISPUTE_RESPONDED. This will protect against typos and make future refactorings much easier, should there ever be a need of changing the allowed values.