Missing submit input for OfficeImporter form

Tested on XWiki 10.8

AFFECTS:

• xwiki:CKEditor.OfficeImporter

ERROR:

• org.xwiki.test.webstandards.CustomDutchWebGuidelinesValidationTest.Validating HTML5 Dutch Web Guidelines validity for: xwiki:CKEditor.OfficeImporter executed with credentials Admin:admin (from org.xwiki.test.webstandards.AllTests)
• ERROR: A form without a submit button has been found. This functionality must not be dependant on a script.

INVALIDATES:

• Dutch Guidelines: validateRpd1s3()
• https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-tools/xwiki-platform-tool-standards-validator/src/main/java/org/xwiki/validator/DutchWebGuidelinesValidator.java#L310-L319
• R-pd.1.3 Do not make the function of the website dependent on optional technology, such as CSS and client-side script: optional technology should complement the information on the site and its use, and should not interfere with access to it if this technology is not supported.

FIXES:

• Add a submit button and not display it for the end user