Collaborators panel UX fixes

[PVince81]

• Accordion / layout confusion: Confusing layout in collaborators UI #1908 => accordion is gone
• Inconsistent role interaction: Role interaction not consistent between collaborators and link share #1907 => roles selection has been improved

From #1186:

• Roles: Need better explanation text
• Roles: Explanation text only needs to be shown in the picker, not again below
• Selected collaborators could be fixed within the autocomplete input field, not below
• Trash bin icon to remove a collaborator maybe not appropriate? => adjusted
• Closing an "edit" dialog without saving may cause human mistakes
• (+) symbol to expand collaborators is not intuitive => removed
• Collaborators can't recognize their role/permissions => Show role + explanation text somewhere

[PVince81]

@wuenschedesign

[PVince81]

[PVince81]

• include private link: Private link icon in "Links" section #2218

[PVince81]

closing in favor of #1186