Convert Tabs to use database

[cooltey]

• Created two database tables: Tab and PageBackStackItem
• Created a custom serializer for the old PageBackStackItem that was stored in Prefs.tabs.
• Move logic to TabHelp
• TODO: ideal way to handle the coroutine process vs. the view updates without having a viewModel in the existing implementation?

Phabricator:
https://phabricator.wikimedia.org/T353656

[cooltey]

A basic refactoring is completed. Pending discussion to see if we need to update the logic of handling tabs vs backstacks.