Allow edit summaries to be up to 767 bytes long

Allow edit summaries to be up to 767 bytes long

This is just the db part. The changes to UI validation code will
come in a dependent patch later.

This changes the max size of various *_comment, *_description
and *_reason fields to be 767 bytes. This size is chosen to
be consistent with the largest we can get away with while
still having covering indexes.

It also unifies the various data types of these fields to
varbinary. (Note, this was previously done and reverted in
r80547, but I don't think the reason for revert applies to
the edit summary fields)

MyISAM has a much smaller max index size, and this wouldn't
work with that storage engine. However, none of these fields
are used in default indexes (only in WMF indexes), so this
should not be an issue for anyone using MyISAM. Postgress
and sqlite already supported the larger edit summary size.

Note: This patch increases min mysql version to 5.0.3.

Bug: T6715
Change-Id: I8558e80a18e4591f07f2c3e80f792ea4435c4e71