Add Action extension for sharing text from other apps to WriteFreely

[AngeloStavrow]

As a user, I would like to type a post in my favourite editor and send it to the WriteFreely app for publishing, or select some text on a website in Safari and be able to create a new post from it.

[AngeloStavrow]

Lots of helpful information in creating an action extension in SwiftUI in this rewrite of Blear.

[AngeloStavrow]

We need to move assorted persistent stores (UserDefaults, Core Data) to a group container so that the iOS app and the action extension can both read from / write to the same place. This means we need to think about two things:

• Migrating persistent stores from the default to the group container
• Migrating user defaults from the standard location to the group container
• Syncing changes to Core Data made by the extension (see this article for ideas)

[AngeloStavrow]

👆 Going to split those tasks off into separate issues that build towards this particular feature.