Add Math display support for qtconsole
Handle sessions where the kernel has been killed
Fix failure message for tests
Fix 404 error when accessing nonexistant session
Correct comment
Add tests and fix some issues

Tests taken from #6360
Pull docs on defining magics out to customisation section
Fix some doc build warnings
Initial version of textbox celltoolbar ui element
Merge pull request #6425 from takluyver/docs-separate-defining-magics

Separate doc on defining magics
Rename chkb to text
Merge pull request #6326 from ccordoba12/math-for-qtconsole

Add Math display support for qtconsole
Merge pull request #6427 from jhamrick/input_ui_generator

Add textbox celltoolbar UI element generator
Merge pull request #6412 from takluyver/sessions-rest-api-fix

Sessions rest api fix