Add needed Imports to the Documentation, Part II

@mjtamlyn
We added missing imports and checked the examples in the following chapters of the documentation (again):

The template layer (@sspross)
Forms (@LeaFin)
The admin (@vkuzma)
Security (@sspross)
Internationalization and localization (@sspross)

All chapters have been checked by a second person, but there may be more missing imports.