Fixed truncated text in OCW descriptions

[Igal Koshevoy]

After yesterday's SQLite to MySQL migration, some columns in OCW had
their text truncated, including important things like the
proposal/session description. This is unfortunate because we received
some good traffic today and were encouraging people to add favorites,
and instead they got truncated data. :(

I've restored the content for the truncated columns from yesterday's
backups, we lost no data, and all seems well again.

Many thanks to Dietrich Ayala for noticing the problem and getting a
hold of Audrey, because she called me and told me about the issue.

The underlying technical issue was that some of our columns used the
"string" SQL type. SQLite interprets "string" as an arbitrary length,
whereas MySQL interprets this as 255 characters. During the migration,
MySQL cheerfully truncated the columns without any warnings, which
thoroughly sucks.

I've migrated the columns that needed more characters from "string" to
"text", restored a backup of the SQLite database, and updated the
affected columns from the backup.

The truncated columns were:
* Proposal/session description
* ScheduleItem excerpt and description
* Snippet description
* Track description

Audrey got in touch at 3:10, I deployed a fix at 3:35, and have spent
the rest of the time since then reading through a complete diff between
the old SQLite database and the current MySQL database to make sure we
didn't lose anything else in the translation. I believe everything else
came through okay.

-igal