ref(issues-stream-assignees): Upgrade assignee dropdown to use <CompactSelect/>

[MichaelSun48]

This PR upgrades the current assignee dropdown menu by replacing it with a <CompactSelect/> implementation. To reduce the scope of this absolutely freaking massive PR, this change only applies to the assignee dropdown on the Issues Stream page and not in the Issue Details page or User Feedback page.

Before:

After:

It also makes the following design tweaks:

• Suggest Assignees, Users, and Teams are now separated into different sections in the assignee list, in that order
• The current assignee, if one exists, is now bubbled to the top of the list, and has a check mark next to its name
• The user's name is no longer bubbled to the top of the list

Open Design Questions:

• Do we want to bubble the user's name to the top of the assignee list?

TODOS:

• [x] Add test file for NewAssigneeSelectorDropdown (placeholder name)
• [x] Rename NewAssigneeSelectorDropdown to something acceptable
• [x] Open invite/request new member modal when (+) Invite Member is clicked

[codecov[bot]]

Bundle Report

Changes will increase total bundle size by 8.14kB :arrow_up:

Bundle name	Size	Change
app-webpack-bundle-array-push	26.55MB	8.14kB :arrow_up:

[scttcper]

i feel like we should still have yourself at the top. that has to be 99% of most assignments

this might need to be behind a flag?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.87500% with 53 lines in your changes are missing coverage. Please review.

Project coverage is 79.99%. Comparing base (44b2e82) to head (e5739fd). Report is 24 commits behind head on master.

:exclamation: Current head e5739fd differs from pull request most recent head f7e5ba1. Consider uploading reports for the commit f7e5ba1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #69639      +/-   ##
==========================================
+ Coverage   79.96%   79.99%   +0.02%     
==========================================
  Files        6495     6496       +1     
  Lines      289760   289925     +165     
  Branches    49922    49958      +36     
==========================================
+ Hits       231720   231924     +204     
- Misses      57628    57633       +5     
+ Partials      412      368      -44     

Files	Coverage Δ
...atic/app/components/deprecatedAssigneeSelector.tsx	100.00% <100.00%> (ø)
static/app/components/stream/group.tsx	55.47% <10.52%> (-6.01%)	:arrow_down:
static/app/components/assigneeSelectorDropdown.tsx	74.28% <74.28%> (ø)

... and 65 files with indirect coverage changes