closes #7822

Use completed_at datetime field as an indication of a performed migration

closes #7805

Previously we had only a Rails validation which ensured poll participation
uniqueness but this adds uniqueness control to the database level, so that
uniqueness is guaranteed even when changing data with avoiding Rails
validations.

closes #7798

This feature only worked on the same pod and was more confusing than
useful.

Some relayables from redmatrix and hubzilla have empty signatures. They
are invalid and break therefore the user data export.

closes #7644

Cleans invalid contacts where the referenced user was deleted from the DB or where
the referenced person owner was deleted from the DB.

closes #7632

When we should have the signature but don't have it, the user data
export fails. There are a few comments from back in 2011 where the
signature is missing.

Also some podmins maybe messed with signatures in their database, which
would also break the exports now.

closes #7637

closes #7630

There are a few old oEmbed caches which have the title saved in binary
(because they contain Chinese characters). This fails with
`ActionView::Template::Error ("å" from ASCII-8BIT to UTF-8)`. Since I
found only very old OEmbed caches with this problem (newest from 2012),
I think we can just remove these. When I create a new oEmbed cache for
the same URL it creates it without `!binary`.

closes #7620

closes #7614

This was added by accident in 4e18f384.

closes #7598

This was a leftover from rails_admin which was removed in #7440.

closes #7597

closes #7580

And cleanup old reshares with deleted root posts

closes #7578

This commit introduces support for AccountMigration federation message
receive. It covers the cases when the new home pod for a user is remote
respective to the recepient pod of the message. It also allows to initiate
migration locally by a podmin from the rails console. This will give the
pods a possibility to understand the account migration event on the
federation level and thus future version which will implement migration
will be backward compatible with the pods starting from this commit.

See #7234

Otherwise Rails 5 would ignore the migration version and create the
tables with bitints as IDs on MySQL.

we released that in 0.5.0.0 in 2015, we do not support skipping majors
anyway, and this is broken in Rails 5, so let's remove this. If people
upgrade from before 0.5.0.0, they have to upgrade via 0.6.0.0, but
that's written in the documenation.

Although this is contrary to rails best-practises, we cannot provide a schema.rb that works for both MySQL and PostgreSQL, so we have no choice. Our migrations are maintained, so it should always be possible to get back to a "clean" database schema anyway.

... this breaks the Rails 5 upgrade, and it's actually no longer needed.
New installations will have the right size anyway, and even if some
older installations miss the migration by not updating for 2 years, it
still doesn't matter since there is no risk that we will ever have
emojis in our migration filenames.

Related to #36

closes #7158

fixes #4319

closes #7118

Turns out that postgres can't handle "0" as boolean, but mysql can
handle "false".

closes #7115#pullrequestreview-699342

Also fixed the "fix public photos"-migration, because it didn't work
with migration-models :/

fixes #6177

Also remove confirm_email_token when unconfirmed_email is removed.

Fixes #7048

closes #7051