Share with owncloud in multiaccount folders only works for the first account created

[SamuAlfageme]

As pointed in #3764 (comment) the Share with ownCloud is troublesome for shared (meaning multiaccount #3764) folders. Couple of issues that should be addressed:

• Right now, only the first user that created the folder sync connection get to share files from the contextual menu. And you get SocketApi: Sending message: "SHARE:NOTSYNCED:/file.txt" until the new user's files that you are trying to share are uploaded to the first user account. (Related: Right-click »Share with ownCloud« not available on folders which have not synced 100% yet #4608)
• An account picker should be implemented to choose which account should originate the share

Also, consider @phil-davis concerns in #3764 (comment)) :

And so the client needs to know which upstream servers have the file - e.g. if it has been just downloaded from account1 and is yet to finish uploading to account2 - so it can present only the valid accounts to share from in the account picker.

[ckamm]

@SamuAlfageme @jturcotte The "one folder, multiple accounts" feature leads to edge cases that we sometimes intentionally don't address (like selective sync, now this one) -- it may be a good idea to pop up a warning when users configure this kind of setup, letting them know that it's a possible, but weakly supported configuration?

As far as I remember there was no pressing need for having the feature.

[TheOneRing]

Multiple accounts for one folder is no longer supported