Implement new style and layout

[LanesGood]

This contains a big commit for style and layout updates. This PR:

• uses DevSeed collecticons in place of image pngs
• Combines the “Add team members” panel with “Select who’s coming” to create a single "Attendees" panel
• Updates layout for a single sidepanel with two collapsable sections: Attendees and Meeting Locations
• Condenses first two columns of each table to a single stacked column for each: Name + Group for attendees, Location + IATA code + Country + Flag for locations
• Collapses the "Group Options" and "Delete" actions into a single "Options" dropdown on each row for the attendees table
  • visible in screenshots below, the group options are present only on members in groups
• Updates theme and styling
• Applies basic responsive views

A few notes:

• I think there is a more elegant way to set id's on the panels for the expand/collapse action - I didn't quite get there
• The Popup is not inheriting the DevSeed theme UI - I'm not sure what the solution is here. Wrapping the Popup component in the ThemeProvider did not fix this fixed!

[nerik]

This is looking amazing 🙏

One thing that I think was problematic with the collapsable panels is that you could easily end up in that "empty" state:

So I tweaked the panel behavior a little bit:

• You can't collapse a panel anymore if it's the only one visible
• The app now opens with only the 'attendees' panel opened. The app seemed like a lot to take in at first load.
• Clicking the 'done' button now opens the 'locations' in its entirety.

Thoughts @LanesGood

[LanesGood]

@nerik thanks for these changes and the improvements to the panel state.

One slight suggested change: what if we had both panels open by default, but have the "Add at least 2 attendees to show results" line in the <Candidates> panel rendered when no results are present? Like the below:

[wrynearson]

Fantastic work @LanesGood! I love the new UI layout and functionality.

Using @nerik's globe map style, I stress-tested this with 694 participants. Co2ordinate handles it very well 😄. It'd be nice to test with your map style.

Co2ordinate.stress.test.mp4

[wrynearson]

However, I did find that the attendees checkbox status wasn't in sync when the select/deselect all members from group X functionality was used.

Co2ordinate.attendee.selection.bug.mp4

[nerik]

@wrynearson Thanks for reporting, tracking these here:
#20 (comment)
#26