Behat tests for image upload from computer, editing images, etc

See `insert-an-image.feature` for a start on this (only "insert from web" is tested so far). Depends on https://github.com/silverstripe/silverstripe-framework/pull/2603 for tests to pass.

Fixes https://github.com/silverstripe/silverstripe-cms/issues/628

```cucumber
@todo
  Scenario: I can insert an image uploaded from my own computer
    Given I press the "Insert Media" button
    And I press the "From your computer" button
    # TODO Figure out how to provide the file
    And I attach the file "testfile.jpg" to "AssetUploadField" with HTML5
    Then the upload field should have successfully uploaded "testfile.jpg"
    When I press the "Insert" button
    Then the "Content" HTML field should contain "testfile.jpg"

  @todo
  Scenario: I can insert an image from the CMS file store
    Given I press the "Insert Media" button
    And I press the "From the CMS" button
    And I select "folder1" in the "Find in Folder" dropdown
    And I select "file1.jpg"
    When I press the "Insert" button
    Then the "Content" HTML field should contain "file1.jpg"

  @todo
  Scenario: I can insert multiple images at once
    Given I press the "Insert Media" button
    And I press the "From the CMS" button
    And I select "folder1" in the "Find in Folder" dropdown
    And I select "file1.jpg"
    And I select "file3.jpg"
    When I press the "Insert" button
    Then the "Content" HTML field should contain "file1.jpg"
    And the "Content" HTML field should contain "file1.jpg"

  @todo
  Scenario: I can edit properties of an image before inserting it
    Given I press the "Insert Media" button
    And I press the "From the CMS" button
    And I select "folder1" in the "Find in Folder" dropdown
    And I select "file1.jpg"
    And I follow "Edit"
    When I fill in "Alternative text (alt)" with "My alt"
    And I press the "Insert" button
    Then the "Content" HTML field should contain "file1.jpg"
    And the "Content" HTML field should contain "My alt"

  @todo
  Scenario: I can edit dimensions of an image before inserting it
    Given I press the "Insert Media" button
    And I press the "From the CMS" button
    And I select "folder1" in the "Find in Folder" dropdown
    And I select "file1.jpg"
    And I follow "Edit"
    When I fill in "Width" with "10"
    When I fill in "Height" with "20"
    And I press the "Insert" button
    Then the "Content" HTML field should contain "<img src=assets/folder1/file1.jpg width=10 height=20>"

  @todo
  Scenario: I can edit dimensions of an existing image
    Given the "page" "About us" contains "<img src=assets/folder1/file1.jpg>"
    And I reload the current page
    When I highlight "<img src=assets/folder1/file1.jpg>" in the "Content" HTML field
    And I press the "Insert Media" button
    Then I should see "file1.jpg"
    When I fill in "Width" with "10"
    When I fill in "Height" with "20"
    And I press the "Insert" button
    Then the "Content" HTML field should contain "<img src=assets/folder1/file1.jpg width=10 height=20>"
```