Custom fields about sumi

[vgwidt]

Frontend will require settings routing to be reworked. I'd like to try a nested router for this.

[vgwidt]

Removing is_select in favor of having it being defined by the field_type. Also added order_index.

[vgwidt]

Would the implementation of custom fields affect how tags will be implemented? I like the idea of having both so that the user can choose what is appropriate. If tags had two levels (like Bookstack), it could eliminate the need for custom fields, but it would have to be implemented in a way that it could be shown as columns, filtered, and added to reports. So should we stick to adding custom fields then a more simple tag system like that of Github? Tags could still have subtags in this case, but without adding support for columns, reports, etc..