fix: membership management button stacking

[gavinbarron]

Closes #2777

Depends on #2779

PR Type

• Bugfix

Description of the changes

stacks buttons in popover vertically
updates button in popover to use the subtle appearance as this provides a visible hover state that more closely aligns with the Teams client
updates layout of member list to use correct padding
removes Button from the ListItem as it was creating a weird experience when navigating with a keyboard where you had to press the arrow key twice for each ListItem
adds styling to show the dismiss icon when navigating with a keyboard
closes the dialog when activating the ListItem for the current user to mimic the behavior in Teams

PR checklist

• Project builds (yarn build) and changes have been tested in at least two supported browsers (Edge + non-Chromium based browser)
• All public APIs (classes, methods, etc) have been documented following the jsdoc syntax
• Stories have been added and existing stories have been tested
• Added appropriate documentation. Docs PR:

• License header has been added to all new source files (yarn setLicense)
• Contains NO breaking changes

Other information

[microsoft-github-policy-service]

Thank you for creating a Pull Request @gavinbarron.

This is a checklist for the PR reviewer(s) to complete before approving and merging this PR:

• I have verified a documentation PR has been linked and is approved (or not applicable)
• I have ran this PR locally and have tested the fix/feature
• I have verified that stories have been added to storybook (or not applicable)
• I have tested existing stories in storybook to verify no regression has occured
• I have tested the solution in at least two browsers (Edge + 1 non-Chromium based browser)

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