Support basic import of space templates

We currently don't import space templates. We can since space templates are in the space exports.

We introduce:

• A "Template space" option with a default value of "Templates", which will, if not empty, import the template there
• A "Produce template providers" option with a defaue value of true, which will produce template providers for these templates

In Confluence, templates contain placeholders that are filled with a cute dialog box on page creation. We don't do anything about this aspect as we deal with such things way differently and don't have feature parity here.