Replace internal/database with base/database

[alovak]

As part of #120 with this PR we are replacing internal/database with base/database package:

• replaced internal/database with base/database
• created separate migration files in /migrations
• updates makefile to use pkger before building binary

There are some issues/concerns:

• base/config can't be used out of the box right now as a replacement for Customers' configuration as it does not support environment variables. It reads config from the file. After we update base/config to work with env variables then we can replace Config in customers.
• we should backup production DB as I have no idea how this new migrations may affect production DB (as all migrations will be run on deploy) 🤷‍♂️ -> to avoid this we can manually create (schema_migrations table and insert value of last migration (16))...
• I don't know how pkger (added into makefile) will work on Windows. Need help with testing this.

P.S. this PR depends on https://github.com/moov-io/base/pull/96

[adamdecaf]

We do nightly backups already. I was planning on renaming the existing DB tables, deploying this (so the new tables are created) and then copying data over.

We kept all the table names, index names, column names/types the same right?

[alovak]

Closing this PR as it's easier for me to open a new one rather to re-base it :(