Abstract Caves, Cavers and Clubs into Models

[anorthall]

At the moment, these fields on a Trip are just text fields. The data should be abstracted into a model which is automatically created at the point of saving the Trip, utilising Django's get_or_create function. A view can then be created to easily bulk update certain cave names or caver names, as well as allowing auto completion of cave names and cavers using select2.

It is important that the Trip add form stays the same as that this process is relatively transparent to users.

This task will involve a significant amount of work to create migrations and update data structures and templates however it will be a significantly improved way to manage the data.

The objects created for "Cavers", "Caves", etc, should be specific to the user.

The implemention of this feature would be important to enable other features, such as #54.