Add invoice prefix input validation

[AlexP11223]

Fixes #390

Limited max length to 15 and added pattern attribute with regex to allow only latin letters and -, _.

When contains not allowed characters, the red border appears:

There are some issues such as that when saving via the button and the prefix is invalid, the page will scroll to it, but not enough, and it will be hidden under the page header.

But during input the red border appears immediately, so it should be good enough for this admin setting.

At first I was thinking to simply delete/replace the not allowed characters during saving on the server side, but it could result in confusion and bad prefixes (not unique across the merchant instances), because the merchant would not notice it after saving + afaik currently there is no good way to show a noticeable error message after saving, and we do not validate anything else like this.