Allow synchronous form answering and spreadsheet editing

[artonge]

Nextcloud (please complete the following information):

• Nextcloud-Version: [e.g. 19.0.0]
• Forms-Version: [e.g. 2.0.0-beta4]

Is your feature request related to a problem? Please describe.

When you open the spreadsheet of a form, the file gets locked, and it prevents people from answering the form.

Describe the solution you'd like

• Store the submitted data in the database
• Try to add the data in the spreadsheet later in a background job

@susnux

[Chartman123]

@artonge could you verify which error message you get, when the file is locked? There's another open issue, where we still don't know exactly where the error comes from (#2067)

We could reproduce the other error now... There shouldn't be a connection to this issue here.