Refactor SessionHelper to use a string template.

Consistency across the helpers is important. Having SessionHelper use
templates like other helpers works to that end.
Use an early return to remove a layer of nesting.
Use short array syntax.
Clean up file headers.
Remove form related tags form HtmlHelper.

The form related HTML generation is all done with string templates now.
These tags are not needed here anymore.
Merge pull request #2965 from cakephp/3.0-session-helper

3.0 Update session helper & clean up HtmlHelper a bit