Bug 28042: Button corrections in OAI set mappings template

Bug 28042: Button corrections in OAI set mappings template

This patch makes minor markup changes in the OAI set mappings template
so that the "Add" button looks consistent with other buttons in the
table.

Also changed: Removed unnecessary inline style attributes.

To test, apply the patch and go to Administration -> OAI sets
configuration

- If necessary, create an OAI set.
- Click the "Actions" button and choose "Define mappings."
- In the table of form fields, confirm that the "Add" button is styled
  similarly to the "Delete" button.
- Confirm that the "Add" button works as expected.

Signed-off-by: Amit Gupta <amitddng135@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>