Implement drag & reorder functionality & new API

[atodorov]

NOTES:

• uses Sortable.js for drag & reorder

• manual sorting button is located after A-Z button in toolbar

• NEW API method TestPlan.update_case_order()

• Manual reorder & API require the testcases.change_testcaseplan
  permission

• toolbarDropdowns() is called from within toolbarEvents() b/c
  it really doesn't make sense to have this as stand-alone function

• toolbarEvents() is moved within the TestCases.filter() callback
  b/c we need the list of test cases to be fully initialized so we
  can get the initial sorting order and use it later

WARNINGS:

• may want to unbind the on-click event for rows if users are
  having problems with the rows expanding while dragging them

• the page doesn't use sortkey for initially displaying the rows
  of test cases because this field isn't available in the API result.

  Specifically sortkey is a field in the TestCasePlan model, which
  is a through model and the .filter() API is querying the TestCase
  model instead.

[RMadjev]

changes proposed

[RMadjev]

LGTM

[atodorov]

Note to self:

Sortable.js seems to not be maintained. There's this alternative:
https://github.com/lukasoppermann/html5sortable

which seems to have all of the functions we need for now and is looking for co-maintainers. We can probably pick it up.

[atodorov]

@asankov, @RMadjev please check the 2nd commit and tell me what you think.

The original sortable.js library states that it is not maintained anymore and I've replaced it with another llibrary which is vanilla JS. The syntax is less handy but we don't use it that much so it is bearable. The changes from one to the other are minimal and straight forward.

The only thing lacking in html5sortable is the animation capabilities but that is offset by the fact that they are actively looking for co-maintainers and we can join them, see:
https://www.npmjs.com/package/html5sortable#looking-for-co-maintainer
lukasoppermann/html5sortable#541