Add UI for setting repeater identity keys

This adds a simple UI in repeater management settings for remotely
setting a repeater's private key. A key can be pasted from an external
key generator, or a random one can be generated here, optionally with a
specified prefix.

If a prefix is specified, searching is done in 30ms batches in order
to keep the UI responsive and show the progress indicator animation
smoothly. Searches can be interrupted. An estimation of search length is
given as expected number of generated key pairs. (A future improvement
would be to observe keys per second performance and give time estimates
based on that.)

When it generates a new private key on your behalf, it can tell you
what the corresponding public key will be. Unfortunately, if you paste
one in from another source, it can't compute the corresponding public
key. This is because the standard Dart crypto library doesn't provide
that capability, nor does it give access to the elliptic curve functions
it implements so we could do it ourselves. Our only recourse would be to
copy the source for all those things, and I didn't think it was worth
it.

This will not check the validity of a pasted key, but the remote
repeater will. It does limit it to the correct length and hexadecimal
charset.

Prefixes are limited to 4 characters. There's no technical reason for
this, but users are unlikely to want more, and I think it would just be
a bad experience to allow more and spin for a very long time. If someone
really wants a longer one, they can use an external generator like they
do now.

Possible TODO: rewrite background processing using isolates.

Closes #142.

lib/models/community.dart

@@ -1,8 +1,8 @@
 import 'dart:convert';
-import 'dart:math';
 import 'dart:typed_data';
 
 import 'package:crypto/crypto.dart' as crypto;
+import 'package:meshcore_open/utils/keys.dart';
 
 import 'channel.dart';
 
@@ -37,11 +37,7 @@ class Community {
 
   /// Generate a new community with a random 32-byte secret
   factory Community.create({required String id, required String name}) {
-    final random = Random.secure();
-    final secret = Uint8List(32);
-    for (int i = 0; i < 32; i++) {
-      secret[i] = random.nextInt(256);
-    }
+    final secret = randomBytes(32);
     return Community(
       id: id,
       name: name,
@@ -210,12 +206,7 @@ class Community {
 
   /// Create a copy of this community with a regenerated random secret
   Community withRegeneratedSecret() {
-    final random = Random.secure();
-    final newSecret = Uint8List(32);
-    for (int i = 0; i < 32; i++) {
-      newSecret[i] = random.nextInt(256);
-    }
-    return withNewSecret(newSecret);
+    return withNewSecret(randomBytes(32));
   }
 
   /// Extract secret from QR data (for updating existing community)