allow alliance extension Fixes #491 (#1314)

## Description:

About 30s before an alliance is about to expire, both players receive a
prompt to extend the alliance. If both players agree the alliance is
extended.

## 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
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

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

evan

src/core/configuration/DefaultConfig.ts

@@ -839,4 +839,8 @@ export class DefaultConfig implements Config {
   defensePostTargettingRange(): number {
     return 75;
   }
+
+  allianceExtensionPromptOffset(): number {
+    return 300; // 30 seconds before expiration
+  }
 }