Caching toots (#809)

* Initial timeline cache implementation

* Fix build/DI errors for caching

* Rename timeline entities tables. Add migration. Add DB scheme file.

* Fix uniqueness problem, change offline strategy, improve mapping

* Try to merge in new statuses, fix bottom loading, fix saving spans.

* Fix reblogs IDs, fix inserting elements from top

* Send one more request to get latest timeline statuses

* Give Timeline placeholders string id. Rewrite Either in Kotlin

* Initial placeholder implementation for caching

* Fix crash on removing overlap statuses

* Migrate counters to long

* Remove unused counters. Add minimal TimelineDAOTest

* Fix bug with placeholder ID

* Update cache in response to events. Refactor TimelineCases

* Fix crash, reduce number of placeholders

* Fix crash, fix filtering, improve placeholder handling

* Fix migration, add 8-9 migration test

* Fix initial timeline update, remove more placeholders

* Add cleanup for old statuses

* Fix cleanup

* Delete ExampleInstrumentedTest

* Improve timeline UX regarding caching

* Fix typos

* Fix initial timeline update

* Cleanup/fix initial timeline update

* Workaround for weird behavior of first post on initial tl update.

* Change counter types back to int

* Clear timeline cache on logout

* Fix loading when timeline is completely empty

* Fix androidx migration issues

* Fix tests

* Apply caching feedback

* Save account emojis to cache

* Fix warnings and bugs

app/src/main/java/com/keylesspalace/tusky/entity/Status.kt

@@ -21,7 +21,7 @@ import java.util.*
 
 data class Status(
         var id: String,
-        var url: String,
+        var url: String?, // not present if it's reblog
         val account: Account,
         @SerializedName("in_reply_to_id") var inReplyToId: String?,
         @SerializedName("in_reply_to_account_id") val inReplyToAccountId: String?,