Remove style guide copy referring to mappings

* Hangover from Transition
Update advice on buttons

* Make it clearer that grey buttons should be the default choice
* Be clearer about what to do with cancel buttons
Add table filter to style guide

* Show example of how to filter a table
* Tweak existing table markup to fix invalid markup and missing
javascript hooks
Merge pull request #35 from alphagov/style-guide-copy

Improve guidance on buttons and tables