[FEATURE] Lock "Connect to server" in Nextcloud settings while connecting

[pbek]

In the Nextcloud settings the "Connect to server" button should not be pressed multiple times until the connection test are done.

[pbek]

26.4.12

• The Connect to server button in the Nextcloud settings is now disabled while a
  connection test is running to prevent starting overlapping requests; changing any
  Nextcloud setting or switching to another cloud connection now aborts the in-flight
  test and immediately re-enables the button (for #3557)