Improve export function

If there is only one file to export, we don't need of a .zip archive.
So it is exported as a simple file (.json or .opml)

See https://github.com/marienfressinaud/FreshRSS/issues/494
Add a test to check presence of Zip archive.

A notification is shown if we cannot use ZipArchive.

See https://github.com/marienfressinaud/FreshRSS/issues/494
Change view import/export if no zip extension

Show a select with only one choice is there is no zip extension on
the server.
Fix typo.

See https://github.com/marienfressinaud/FreshRSS/issues/494