fix accessibility issues with wiki sidebars image tab

It's now much easier to navigate and select images with the keyboard in
the 'images' tab of the wiki sidebar. Screenreader labels are now more
descriptive and when you click on 'upload a new image' it focuses you to
the first item automatically. Also, when searching flickr for photos,
you are now able to easily tab through the search results.

fixes CNVS-8998

Test Plan
1. Go to a page with a wikisidebar (Pages -> Select page -> Edit)
2. Use the keyboard to navigate to the images tab. Pressing the tab key
from the images tab should take you to the 'upload new image' link.
3.Hit enter to open and toggle the link. It should focus you
automatically on the select folder option.
4. When finding an image via the 'Find images on flickr creative
commons' link you should be able to use the keyboard to search and
navigate (via with tabs) the search results.
5. You should be able to insert an image into the editor after searching
by hitting the enter key.
6. Do steps 1 - 4 with the screenreader for detailed labels and to
ensure it can be navigated.

Change-Id: I714c19a87ae7f88525fc35aab37e4e057914f906
Reviewed-on: https://gerrit.instructure.com/28108
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>