Fix duplicate PubSub subscription

Fixes

• Fixes a duplicate subscription to the PubSub topic by only subscribing on the second/stateful LiveView mount.
  Thanks to @Maxim-Filimonov for pointing this out!

Bugfix: Make Store topic unique per LiveView

Fixes

Previously, the store PubSub topic was hard-coded to always be live_ex_store. This meant that if multiple LiveViews initialised the store, they would overwrite each others state since all state updates from all LiveViews were broadcasted on the same topic. This is now fixed by extending the live_ex_store_topic with the PID of the LiveView. This way, the store topic will always be unique per LiveView.

Breaking Changes

• The topic-option was renamed to live_ex_store_topic. This way, you can set a topic assign on your socket without overwriting the LiveEx internal topic assign.

Update dependencies, replace Poison with Jason, and small changes.

• Update to LiveView 1.15.7
• Replace Poison with Jason
• Make the PubSub name customisable
• Add Dialyzer and Credo