restrict problematic flags fixes #1690 (#1805)

## Description:

Some flags are offensive, so don't allow them to be selected by users.

I think it's fine for nations continue to use the flags.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

src/client/data/countries.json

@@ -484,7 +484,8 @@
   {
     "code": "Confederate States",
     "continent": "North America",
-    "name": "Confederate States"
+    "name": "Confederate States",
+    "restricted": true
   },
   {
     "code": "1_Connacht",
@@ -702,7 +703,8 @@
   {
     "code": "Fascist Spain",
     "continent": "Europe",
-    "name": "Fascist Spain"
+    "name": "Fascist Spain",
+    "restricted": true
   },
   {
     "code": "fm",
@@ -805,7 +807,8 @@
   {
     "code": "German Empire",
     "continent": "Europe",
-    "name": "German Empire"
+    "name": "German Empire",
+    "restricted": true
   },
   {
     "code": "de",