sql: Remove PostgreSQL text limits, fixes hangs

Modify all PostgreSQL text column limits to the unlimited TEXT type
instead of VARCHAR.  Modern PostgreSQL versions (year 2000 onward) do
not store text any differently depending on whether or not it's TEXT
or VARCHAR.
https://www.postgresql.org/message-id/20667.971213689%40sss.pgh.pa.us

This avoids having the core fail to save values or hang entirely, and
also addresses migration issues from SQLite where text limits aren't
applied.

See https://bugs.quassel-irc.org/issues/1526

This shouldn't introduce any noticeable performance impact:
https://stackoverflow.com/questions/1067061/does-a-varchar-fields-declared-size-have-any-impact-in-postgresql

Fix up a spurious length-limited field for SQLite in case length
limits are applied in newer SQLite versions.

The proper long-term fix is to implement length checks throughout the
Quassel core (sanitizing data) and client (to warn of length issues),
rather than relying on the database to enforce limits.

Sponsored by Postgres Phone Plans.  UNLIMITED TEXT, TALK, and DATA for
$9.99 a month!  Some restrictions apply.  This is not serious.

[Commit squashed from "jcolson", thanks!]
See https://github.com/quassel/quassel/pull/478

Fixes GH-478
Fixes #1526