Update documentation of nginx configuration

- Avoid using "if" to check for file existence (use try_files instead)
- Replicate the behavior of the .htaccess files
- TODO: get static error pages to work
API Require ADMIN for ?showtemplate=1
Merge pull request #1181 from chillu/pulls/showtemplate-admin

API Require ADMIN for ?showtemplate=1 (2.4)
Nginx docs for denying composer file access (fixes #8011)
BUG Undefined `$allowed_actions` overrides parent definitions, stricter handling of $allowed_actions on Extension

Controller (and subclasses) failed to enforce $allowed_action restrictions
on parent classes if a child class didn't have it explicitly defined.

Controllers which are extended with $allowed_actions (through an Extension)
now deny access to methods defined on the controller, unless this class also has them in its own
$allowed_actions definition.
Improved docs on $allowed_actions

Added section to "Controllers" and "Form" topics,
added $allowed_actions definitions to all controller examples
BUGFIX Keep Member.PasswordEncryption setting on empty passwords

This will prevent empty passwords to set the encryption to 'none',
which in turn will store any subsequent password changes in cleartext.
Reproduceable e.g. with ConfirmedPasswordField and setCanBeEmpty(true).
Add ContentController->handleWidget() to $allowed_actions

Required by recent $allowed_actions security fix
Updated changelog
Merge remote-tracking branch 'origin/2.4' into post-2.4
BUG Transaction stub methods for better cross 2.x and 3.x compat

The transactionStart() naming is 3.x, and used by some modules
which are otherwise still 2.x compatible.

Specifically, this was added to avoid branching the payment module
into separate 2.x and 3.x compatible branches.
Merge remote-tracking branch 'origin/2.4' into post-2.4