fix(fxa-admin-panel) use clearer icons to indicate permission status

[millmason]

Because

• Currently, a user can be confused as to whether they have a given permission or not, because the negative symbol is x. We use clearer positive/negative symbols to indicate permission status.

This pull request

• Replaces the symbols with more vivid unicode symbols (U+2705: ✅ and U-274C: ❌)

Issue that this pull request solves

Closes: #12743

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

Screenshots (Optional)

Before (user has permissions):

Before (user does not have permissions):

After (user has permissions):

After (user does not have permissions):

To test:
With the fxa-admin-panel app running in local, visit http://localhost:8091/permissions and observe the permissions for your user. I don't know how to change the permissions for the logged in user, but to view the opposite of whatever permissions you're seeing, switch the values in the ternary here like so: val: flag.enabled ? '❌' : '✅',