Don't start a session if it's known to be empty.

If an app only reads/checks the session there's no need to start a
session to know that the read/checked session value is empty.

Fixes #1981
Avoid code duplication.
Added regex support to Validation::mimeType().
Fix link in comment
Merge pull request #2743 from mattalexx/fix-comment

Fix link in comment
Allow empty values in checkboxes.

Allow the checkbox value attribute to be empty. This is required to make
checkboxes with a value of 0.

Fixes #2717
Fix plugin path filtering when directories do not exist.

When paths do not exist we should correctly remove them from the output
paths.

Fixes #2748
Fix: Changing boolean to integer for Postgres

Closes #2751, CakeDC/migrations#149
Merge pull request #2752 from rchavik/2.4-postgres-alterschema

Fix: Changing boolean to integer for Postgres
Merge pull request #2693 from ADmad/2.5-session-start

Don't start a session if it's known to be empty.
Add '@' to the list of characters removed from radio/checkbox ID.

This character can easily show up in email addresses and can cause
invalid HTML.

Refs #2733
BC fix for checkbox ids

Revert to previous behavior for id generation of checkboxes, This commit
can be reverted for 2.5+ is desired

Fixes #2733
Merge pull request #2766 from AD7six/2.4.x-bc-form-fix

BC fix for checkbox ids
Added missing 'Session'-Compomnent in bake

If a Controller was baked interactively and one want to use Session flash messages, the required 'Session'-Component wasn't added to the $components-Array
removed indentation
fixed coding standard errors
Fix bake templates to generate functioning code.

Closes #2776
Merge branch 'master' into 2.5
Fixed HTTP Status code when ajaxLogin is set
Clarified ACL node lookup failed error.

Original error was ambiguous in stating whether the ARO node lookup failed or the ACO node lookup failed.