Setup more flexible templates for groups, tabs, and repeatable fields

Logic and templates are too closely intertwined in a few places (e.g. [`Fieldmanager_Field::element_markup()`](https://github.com/alleyinteractive/wordpress-fieldmanager/blob/623340e9a2f8a51e6969423d9961c74207a34c3d/php/class-fieldmanager-field.php#L333-L427), [`Fieldmanager_Field:: single_element_markup()`](https://github.com/alleyinteractive/wordpress-fieldmanager/blob/623340e9a2f8a51e6969423d9961c74207a34c3d/php/class-fieldmanager-field.php#L429-L499) and [`Fieldmanager_Group::form_element()`](https://github.com/alleyinteractive/wordpress-fieldmanager/blob/623340e9a2f8a51e6969423d9961c74207a34c3d/php/class-fieldmanager-group.php#L112-L206)).

I propose that we move all the templating to `Fieldmanager_Context` and change these methods to return the data that they processed. This way, a context can more easily override the generated HTML to better suit the screen (e.g. term and user forms).