Add section template controls in Email Settings

[obruchez]

1. When using an All Control Values template parameter, all controls will be included, including those in section templates, but it's not possible to select controls from section templates under "Exclude from All Control Values".

2. For headers and template parameters, it's also not possible to select controls from section templates when selecting a Control Value. Manually adding a section template control to the form definition works, though (e.g. <header name="X-Custom" type="control-value" section-template="section-3">control-1</header>).