Fix seed admin password too short for Devise validation

Generate a random 12-character alphanumeric password instead of the
hardcoded 'admin' (5 chars) which fails Devise's minimum length
requirement of 8 characters. The generated password is printed to
stdout so the operator knows what to use.

Fixes #71

db/seeds.rb

@@ -18,9 +18,10 @@
 User.transaction do
   break if User.find_by status: :admin
 
-  User.create! email: Rails.configuration.admin, password: 'admin', status: :admin do |user|
+  password = SecureRandom.alphanumeric(12)
+  User.create! email: Rails.configuration.admin, password: password, status: :admin do |user|
     user.skip_confirmation!
-    print "Creating #{user.status} account '#{user.email}' with password '#{user.password}'..."
+    print "Creating #{user.status} account '#{user.email}' with password '#{password}'..."
   end
   puts "done."