erp5_document_scanner: prevent crashing the UI

Clicking on a button tag with an embedded img tag crashes everything.
The right way to fix this is probably to drop the img tag and use a CSS background url instead.
10 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

02:11:31

passed ERP5.PerformanceTest-Master

00:34:05

failed ERP5.UnitTest-Master

04:48:57

passed SlapOS.Eggs.UnitTest-Master.Python2

01:05:41

passed SlapOS.Eggs.UnitTest-Master.Python3

00:24:34

passed SlapOS.Eggs.UnitTest-Master.Python2

01:01:35

passed SlapOS.Eggs.UnitTest-Master.Python2

00:43:59

passed SlapOS.Eggs.UnitTest-Master.Python2

00:38:33

passed SlapOS.Eggs.UnitTest-Master.Python3

00:20:03

passed SlapOS.Eggs.UnitTest-Master.Python3

00:16:14