Bug 34034: Move translatable strings out of opac-bottom.inc: OverDrive and OpenLibrary

Bug 34034: Move translatable strings out of opac-bottom.inc: OverDrive and OpenLibrary

This patch removes some string variable definitions from
opac-bottom.inc. The strings can now be picked up for translation from
the corresponding <script> block.

To test you must enable OpenLibrarySearch and enter OverDrive
credentials to enable OverDrive search integration.

- Perform a search in the OPAC.
- As the results page loads you should see at the top "Searching
  OverDrive..." and "Searching OpenLibrary..."
- When the searches complete you should see "Found X results in the
  library's Y collection" for each service.
- Try to trigger an error message by entering the wrong (non-zero) value
  for the OverDriveLibraryID preference.
- The search results page should show "Error searching OverDrive
  collection"

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>