Alert Profiles does not validate IP addresses in filter expressions

[lunkwill42]

The Alert Profiles UI does not validate the input of IP address filters, meaning any string can be entered as an IP address value.

The Alert Engine takes for granted that the filter value is semantically correct, and uses it directly when querying PostgreSQL, which results in PostgreSQL errors that are logged like this (every 30 seconds into eternity or until the filter is no longer present in a profile, whichever comes first):

2019-02-28 11:46:46,266 [ERROR] [nav.alertengine] Database error, closing the DB connection just in case:
invalid input syntax for type inet: "shit"
LINE 1: ...L AND "alertq"."alertqid" = 211352 AND (netbox.ip = 'shit'))

[johannaengland]

Closed by #2667