Fixes #5605 alembic migration errors on Focal

[kushaldas]

Status

Ready for review

Description of Changes

Fixes #5605

Due to SQLite3 upgrade in Focal, our previous migration scripts
were failing.

From SQLite website https://sqlite.org/lang_altertable.html:

enhanced in versions 3.25.0 (2018-09-15) and 3.26.0 (2018-12-01) in order to
carry the rename operation forward into triggers and views that reference the
renamed table. This is considered an improvement. Applications that depend on
the older (and arguably buggy) behavior can use the PRAGMA legacy_alter_table=ON
statement or the SQLITE_DBCONFIG_LEGACY_ALTER_TABLE configuration parameter on
sqlite3_db_config() interface to make ALTER TABLE RENAME behave as it did prior
to version 3.25.0.

As @rmol suggestied, this PR is only using the PRAGMA calls where we are doing rename_table operations.

Testing

• make test should pass all tests
• make test-focal (after rebasing) on the dev_focal branch should not have any Alembic test failures
• CI should be green

Deployment

Any special considerations for deployment? Consider both:

1. Upgrading existing production instances.
2. New installs.

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

If you made changes to securedrop-admin:

• Linting and tests (make -C admin test) pass in the admin development container

If you made changes to the system configuration:

• Configuration tests pass

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

Choose one of the following:

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation
• These changes do not require documentation

If you added or updated a code dependency:

Choose one of the following:

• I have performed a diff review and pasted the contents to the packaging wiki
• I would like someone else to do the diff review

[rmol]

Looks good to me, of course.