Allow keeping proxy tags when proxying messages.

Required database migration:

Closes #75.

PluralKit.Core/Stores.cs

@@ -495,7 +495,7 @@ namespace PluralKit {
 
         public async Task SaveMember(PKMember member) {
             using (var conn = await _conn.Obtain())
-                await conn.ExecuteAsync("update members set name = @Name, display_name = @DisplayName, description = @Description, color = @Color, avatar_url = @AvatarUrl, birthday = @Birthday, pronouns = @Pronouns, proxy_tags = @ProxyTags where id = @Id", member);
+                await conn.ExecuteAsync("update members set name = @Name, display_name = @DisplayName, description = @Description, color = @Color, avatar_url = @AvatarUrl, birthday = @Birthday, pronouns = @Pronouns, proxy_tags = @ProxyTags, keep_proxy = @KeepProxy where id = @Id", member);
 
             _logger.Information("Updated member {@Member}", member);
         }