Add Toast to Add Collaborator Floating Action Button Long Click

[theck13]

Fix

Add a toast message to the Add Collaborator floating action button long click on the Collaborators screen. The behavior and test mimic the existing Add Tag floating action button long click on the Edit Tags screen.
See the videos below for illustration.

Videos

Before	After
1752_before.mp4	1752_after.mp4

Relates: #1752

Test

1. Tap any note in list.
2. Tap ellipsis/overflow button in top app bar.
3. Tap Collaborators item in menu.
4. Long-press Add Collaborator floating action button.
5. Notice Add Collaborator toast.

Review

Only one developer is required to review these changes, but anyone can perform the review.

Release

These changes do not require release notes.

[mzorz]

Nice!