Bug 28537: Improve HTML generated by OverDrive integration

Bug 28537: Improve HTML generated by OverDrive integration

This patch makes changes to the way HTML is generated when showing
output from OverDrive integration:

- Buttons have Bootstrap button classes
- Layout of information is improved.
- Handling of cover image thumbnails is improved, especially in cases
  where images were much larger than expected.
- An OverDrive-specific CSS file is added.

To test you must have valid credentials entered in system preferences
for the OverDrive API. You should test using a patron who has holds and
current checkouts in OverDrive.

Apply the patch and log in to the OPAC.

- On the "Your summary" page, open the "OverDrive Account" tab.
  - Log in to your OverDrive account.
  - The contents of the tab should display well, with item
    thumbnails showing in a column to the left, and item information
    showing to the right.
  - Controls should be styled with standard Bootstrap style.
  - Confirm that operations work correctly: Acessing titles, checking
    in, canceling holds, etc.

- Perform a catalog search which will return results also found in your
  OverDrive collection.
  - Open the OverDrive results page.
  - The page should display correctly, especially thumbnails.
  - Confirm that operations work correctly: Checking out and placing
    holds.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>