Add more best practices to code templates

[david]

We need to have more best practices in our standard code templates to keep support volume down in the long run and allow people to get started more quickly (not to mention tutorials, magazine articles, screencasts that do not need to explain some of this very basic stuff first):

• standard layout and slot layout
• check against is_slot in setupHtml()
• strict validation mode even in dev envs?

[fnordfish@…]

- when and how to use nested routes - what output types _can do_ (htmlPrint, default assigns, ...) - validation (how (not) to use operator-validation, how to use 'dependencies'), when shall i write my own, how should they look

[anonymous]

* form population (injecting default values, error-msgs, and combined)

[v-dogg]

• Production-ready exception template (i.e. something that doesn't disclose all the exception + environment details to the world. Could be something simple like plain text "Application Error".) All this just because people seem to be too lazy to replace the default template in production.

[david]

(In [3304]) added more best practices to code templates and generated system actions, closes #910 and #911

[david]

(In [3305]) send http 401 on "forbidden" pages, refs #910 and #911

[david]

(In [3306]) send http 503 on "unavailable" pages, refs #910 and #911

[david]

(In [3310]) renamed templates/exception to templates/exceptions for new projects and fixed an mkdir typo for that directory in build.xml, refs #910