Ability to edit specified content type and remove batch size setting

[iuravic]

Describe the solution you'd like
The plugin settings already display a read-only input field with post types which will get converted, and is presently not only read-only, but hardcoded to 'post','page' too. Replace this non-editable post types input with an editable input, which lets the user add custom post types too.

Also remove the input field for the batch size setting of 100 -- this tested and tried recommended number should simply be hardcoded as a const.