Add webUI acceptance tests for sharing with multiple users at once #40178
Description
PR #40155 adds the ability to share with multiple users at the same time.
For example, Alice could paste Brian,Carol into the share-with text box, and the UI will suggest sharing with both of them.
The multiple-sharing is only with users. To share with groups, you can only do one at a time.
Group names can have a comma in them. So it is possible to have a group with the exact name Brian,Carol. When that happens, then the UI should suggest 2 options for sharing - share with the group Brian,Carol or share with each of the users Brian and Carol.
If a string in the comma-separated list is not a valid user then it should be omitted from the list of people to share with. Paste in Brian,unknown,Carol and the UI should suggest sharing with only Brian and Carol.
Add some automated webUI test scenarios to cover the basic behavior, so that we don't get accidental regressions in future.
