Fix behavior of _ when searching for DAGs

[jedcunningham]

On the homepage, when you search for DAGs, _ wasn't behaving as most people might expect since it is a single character wildcard in SQL. We will automatically escape it now.

[ashb]

Hmmm, there's something built in to SQLA for this https://docs.sqlalchemy.org/en/14/orm/internals.html#sqlalchemy.orm.PropComparator.like.params.escape

Oh, it's for this:

To match a literal underscore or percent sign without matching other characters, the respective character in pattern must be preceded by the escape character. The default escape character is the backslash but a different one can be selected by using the ESCAPE clause. To match the escape character itself, write two escape characters.