Bug 20239: Fix spelling on authority linker plugin

Bug 20239: Fix spelling on authority linker plugin

This patch corrects the capitalization of the "Choose" link in the
pop-up window for selecting an authority record to link to a MARC field.

This patch also converts the link, and the "Edit authority" link, to
Bootstrap buttons.

To test:
- Create a new record
- Go to the 100 field
- Click on plugin link behind $a
- Look for an existing authoriy in your system
- In the table of search results, confirm that the "Choose" and "Edit
  authority" links look correct and work correctly.

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>