Add support for archiving conversations

Archiving messages currently acts like recycle bin in
Simple Gallery, meaning that after 30 days, conversations
will be deleted permanently. Any updates to the conversation
(new messages) removes it from archive.

This closes #177

app/src/main/kotlin/com/simplemobiletools/smsmessenger/models/ArchivedConversation.kt

@@ -0,0 +1,15 @@
+package com.simplemobiletools.smsmessenger.models
+
+import androidx.room.ColumnInfo
+import androidx.room.Entity
+import androidx.room.Index
+import androidx.room.PrimaryKey
+
+@Entity(
+    tableName = "archived_conversations",
+    indices = [(Index(value = ["thread_id"], unique = true))]
+)
+data class ArchivedConversation(
+    @PrimaryKey @ColumnInfo(name = "thread_id") var threadId: Long,
+    @ColumnInfo(name = "deleted_ts") var deletedTs: Long
+)