common: Allow skipping negotiation of IRCv3 caps

Add "skipCaps" to Network, storing a list of IRCv3 capabilities to
skip during capability negotiation.  This allows selectively disabling
capabilities on specific IRC networks.  Some networks might not behave
as expected, and some IRC features (e.g. "echo-message" support) are
enough of a departure from traditional IRC behavior that users may
want to turn it off.

Add the necessary migrations and setup to SQLite and PostgreSQL.

Add the "SkipIrcCaps" feature flag to signal protocol support.

Fix up a comment in CoreNetwork referring to an old variable name.

Update the SQL README to mention the Serializers protocol checks.
Missing this caused me a few hours of confusion over why the protocol
change wasn't being received :)