Write new keyboard steps for Behat

Our current behat steps relating to keyboard are both unclear, and incorrect. We should try to improve them. Example issues include:

1. many of the steps require you press a key in a field, but we should be making these separate steps. It's perfectly legitimate to press keys without being in a field (i.e. keyboard navigation)
2. our current steps require you to know the ascii key char, which leads to confusing steps
3. we use $node->key* methods in several places when we should perhaps be using a standardised API

This issue proposes to introduce new steps:

1. I press the [key] key where key includes:
   1. up
   2. down
   3. left
   4. right
   5. pageup
   6. pagedown
   7. home
   8. end
   9. insert
   10. escape
   11. enter
   12. tab
   13. space
2. I press the [modifier][key] key where modifier includes:
   1. shift
   2. alt
   3. control
   4. meta
   5. a combation of these keys joined with +

Example conversions of these steps:

-    And I press key "27" in the field "Select users to create learning plans for"

+    And I press the escape key

This patch also introduces a new behat_base::type_keys() function which is used by the new step, but can also be used by areas which do not implement a context (i.e. form field handlers)