template-import: Add format check to the import file upload

[FrederikSchlemmer]

This pull request adds a check before accessing the uploaded import file. This verifies that the uploaded file matches the needed file format to perform an import. This behavior is verified by an implemented test.

Closes #355

[czarnecki]

Checking if the filename ends with "xls" or "xlsx" isn't enough to ensure that the file is a valid Spreadsheet file. I could just upload an empty file with the name "xlsx" and it would trigger either the same or a similar exception as before. So we should either verify that it's a valid file beforehand, or somehow inform the Page that it's an invalid file.

[tinne]

When I now start an import to an already existing template, the template is duplicated under the same name instead of (as happened before) the file in the already existing template being swapped. Also, the same template name should be allowed only once per project.

Regarding file recognition:

• .xslt is accepted, which is ok, but not being recognized by error message (may be an undocumented feature, so all right), but
• .xslm is sometimes accepted and sometimes rejected, and I do not understand why.
• .xls is rejected when it is in fact CSV, which is ok, but possibly deserves an error of its own
• uploading real old .xls files lead a new error (Caused by: java.lang.ClassCastException: org.apache.poi.hssf.usermodel.HSSFWorkbook cannot be cast to org.apache.poi.xssf.usermodel.XSSFWorkbook), so either handle this or remove .xls support

Everything else seems to be alright.

As I accidently already tested the ticket, you might consider moving the ticket to project 2 as well.