refactor: rework hooks and self-service flow completion

[aeneasr]

This patch focuses on refactoring how self-service flows terminate and
changes how hooks behave and when they are executed.

Before this patch, it was not clear whether hooks run before or
after an identity is persisted. This caused problems with multiple
writes on the HTTP ResponseWriter and other bugs.

This patch removes certain hooks from after login, registration, and profile flows.
Per default, these flows now respond with an appropriate payload (
redirect for browsers, JSON for API clients) and deprecate
the redirect hook. This patch includes documentation which explains
how these hooks work now.

Additionally, the documentation was updated. Especially the sections
about hooks have been refactored. The login and user registration docs
have been updated to reflect the latest changes as well.

Also, some other minor, cosmetic, changes to the documentation have been made.

Closes #348
See #347
See #179
Closes #51
Closes #50
Closes #31

BREAKING CHANGE: Please remove the redirect hook from both login,
registration, and settings after configuration. Please remove
the session hook from your login after configuration. Hooks
have moved down a level and are now configured at
selfservice.<login|registration|settings>.<after|before>.hooks
instead of
selfservice.<login|registration|settings>.<after|before>.hooks.
Hooks are now identified by hook: instead of job:. Please
rename those sections accordingly.