Add hooks to inject new template types

[cairocoder01]

This refactors the magic link template types (single record and list of sub-assigned contacts) to be initialized using hooks. This will allow developers to create their own template types from other plugins and inject them into the list of available template types.

The current use case that prompted this is to create a template type from my team module plugin. We want to create a template that is all of the contacts that are assigned to the team that I am on. That should be created in the team module plugin, but there isn't a way for that to interact with this plugin at the moment.

Filter: dt_magic_link_template_types

Creates the list of available template types so that a dev can add the name of their template type to the list in the admin

Action: dt_magic_link_template_load

Used to instantiate a magic link class that will display the magic link on the front-end.

Following this being merged, we could add a sample of this to the starter template so that it's easy for devs to see how to do this.

For help in reviewing, the two new files in the templates folder are direct copies of the original magic-link-templates.php file that held the code for those two templates. Minimal code was changed in there besides adding the hooks at the top and changing some private properties to protected so that the list-sub-assigned could inherit the single-record and share code.

[corsacca]

This looks good.
Thank you @cairocoder01 for paving the way!