Upload all existing files on auto upload

[ShayGus]

When initially selecting a folder for auto upload, add the ability to enable the initial upload of all existing files

[nextcloud-android-bot]

GitMate.io thinks possibly related issues are #1295 (Auto Upload: uploads 0b files), #2812 (Add "upload existing"-option to autoupload), #1141 (Auto upload uploads existing files causing duplicates), #1736 (Auto upload overwrites existing files), and #1290 (Auto upload light ).

[AndyScherzinger]

Duplicate of #2812 / #285
with PR/discussion here #2873

[AndyScherzinger]

to be released with 3.11. via #4788