define a readonly acceess

[herrfinke]

Expected behaviour

At the moment we can only block access to files/folders. But would like to have some files, which can be read by everyone but not modified or deleted.
similar to issue #149

Halfway workaround at the moment

we can use the "filesize-at-upload" option and forbid all uploads below 100GB (just a random large number)
but this will not work if a document was edited online (e.g onlyoffice).

Possible implementation

adding options in the condition-dropdowns like:

• if modified
• if renamed
• if deleted

[herrfinke]

sorry, duplicate of #151