Implement new policy: The database version number is always even (#4128)

Posted this as issue #3999 before. The reasoning is personal experiments
and forks may add database fields and must bump the database number to
do so, but this causes massive merge difficulties when Tusky then
inevitably itself bumps the number. To alleviate this, Tusky official
should use only even database numbers, so odd versions are available for
third party scribbling.

There was little discussion positive or negative in #3999 (one proposal
we switch to a date-based number system, which would work but also could
be unnecessarily complicated). With PR #4115 we now have to make a
decision because that's the first post-proposal PR to bump the database
number odd. So, since I see no outright objections, I'd like to
implement this.

@connyduck suggested the best way to implement the proposal would be to
add a comment to the version number's home in AppDatabase.java.

Co-authored-by: Konrad Pozniak <connyduck@users.noreply.github.com>

app/src/main/java/com/keylesspalace/tusky/db/AppDatabase.java

@@ -42,6 +42,8 @@ import java.io.File;
         TimelineAccountEntity.class,
         ConversationEntity.class
     },
+    // Note: Starting with version 54, database versions in Tusky are always even.
+    // This is to reserve odd version numbers for use by forks.
     version = 56,
     autoMigrations = {
         @AutoMigration(from = 48, to = 49),