TUVoteInfo: generalize Quorum

SQLite does not support native DECIMAL columns, and for that
reason, we had to switch to using Strings that can hold the data
in the case we are using sqlite.

This commit sets the TUVoteInfo model up in a generic way, that
it always converts to string when setting Quorum (OK for DECIMAL)
and always converts to float when getting Quorum.

This way, we can treat TUVoteInfo.Quorum as the same thing
everywhere.

Signed-off-by: Kevin Morris <kevr@0cost.org>