Assorted db changes (#333)

* postgres: use transaction-scoped sql for upgrade

* database: combine fixGuild and addGuild into getGuild

* postgres, sqlite: simplify upgrade()

* allow running commands in DMs without prefix

this functionality was broken in
16095c0256 but is now fixed

* allow running esmBot without a database

Before this change, the only way to run esmBot without a database is to use the
dummy database driver which is broken but fails silently. THis can lead to a
confusing user experience. For instance, using `&command disable` with the
dummy database driver will tell you that the command has been disabled even
though it has not been.

This change adds support for running esmBot with no database driver by leaving
the DB= config option empty, and explicitly telling the user that some
functionality is now unavailable rather than failing silently like the dummy
driver.

* remove dummy database driver

docs/config.md

@@ -7,7 +7,7 @@ To make managing environment variables easier, an example `.env` file is include
 ### Required
 - `NODE_ENV`: Used for tuning the bot to different environments. If you don't know what to set it to, leave it as is.
 - `TOKEN`: Your bot's token. You can find this [here](https://discord.com/developers/applications) under your application's Bot tab.
-- `DB`: The database connection string. By default the `sqlite` and `postgresql` protocols are available, but this can be expanded by putting proper DB driver scripts into `utils/database/`. You can also set this to `dummy` to make the bot not use a database at all.
+- `DB`: The database connection string. By default the `sqlite` and `postgresql` protocols are available, but this can be expanded by putting proper DB driver scripts into `utils/database/`.
 - `OWNER`: Your Discord user ID. This is used for granting yourself access to certain management commands. Adding multiple users is supported by separating the IDs with a comma; however, this is not recommended for security purposes.
 - `PREFIX`: The bot's default command prefix for classic commands. Note that servers can set their own individual prefixes via the `prefix` command.
 
@@ -75,4 +75,4 @@ The JSON-based configuration files are located in `config/`.
     // Note: instances must support getting results over JSON
   ]
 }
-```
+```