Add ability to customize search button

Manual test in FOLIO test http://folio-testing.aws.indexdata.com/, using Chrome.

I can verify the two new buttons: '+ Add Sponsor' and '+ Add proxy'.
All looks good, and I'll close the issue as done.

I assumed the commit would be enough here since it's a simple change but you are right I should at least add a little description. Here are more details:

The previous implementation worked well in cases where the search button was attached to the text field (with a search icon and left radius). We are now running into cases where a standalone button is needed.

This is a small cosmetic change in order to be able to change the button label and style. This work is needed for buttons in UIU-104 ("add sponsor", "add proxy").

Customise it in what way?