Remove redundant channel column

[soapy1]

Fixes #994

Description

The primary focus of this pr is to remove the channel_id column and channel association in the conda_package_build table. This is redundant information, also available in the associated package. In order to get channel information from a conda_package_build users may join the conda_package table and conda_package_build table. For example:

from conda_store_server._internal import orm

db.query(orm.CondaPackageBuild)
        .join(orm.CondaPackage)
        .filter(orm.CondaPackage.channel_id == 1)

Things changed in this PR:

• removal of the channel_id column in the conda_package_build table
• a migration script. It does 2 things:
  • updates the conda_package_build to have the correct package associated with each row. See Update conda-package-build causes an integrity error on conda_package_build table #961 for more details about this issue
  • updates the database table column + unique constraint
• updates the docs for how to write a migration
• adds a dependency on pytest-alembic
  
• sets up running tests for alembic migrations

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

Additional Info

Part of the migration is to correct some of the conda_package_build entries that have the wrong conda_package associated with them. Since this needs to sort thru all elements of this table, this migration takes a while to run. Users should probably opt to run this migration directly with alembic.

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[peytondmurray]

At least partially address #733 with the addition of pytest-alembic.

[peytondmurray]

This one seemed like a bit of a tricky migration. The changes look great, thank you for this! Only a few minor suggestions here.