Add config forms for source table

[julianbrost]

Currently, the source table has to be populated manually by the user by issuing insert statements to the database. As the daemon loads most of it's configuration from the database, additional source configuration should also be managed from the web module.

With upcoming PRs in the daemon, there will be extensions to the source table, that should be possible to configure from web. This issue is kind of a heads-up notice, the details will be finalized in Icinga/icinga-notifications#132 and Icinga/icinga-notifications#112 but roughly are the following:

• type = 'icinga2' will become special in the way that it enables extra functionality in the daemon to pull events from the Icinga 2 API. Therefore, extra columns will be added for the API URL/user/password/TLS config that should be populated only for this type.
• All other types will use the existing HTTP listener that allow submitting events via HTTP POST request. This will be protected with basic auth and therefore, a column for a password hash (bcrypt) will be added (no column for the username, that's basically the source ID).

[julianbrost]

The schema should be mostly finalized in https://github.com/Icinga/icinga-notifications/blob/icinga2-source/schema/pgsql/upgrades/022.sql by now, with the exception that an additional column for the endpoint name will probably be added as Icinga 2 doesn't really encourage using a resolvable hostname as the endpoint name, but currently, that would be required to be able to verify the certificate (see Icinga/icinga-notifications#112 (comment) for a little more details).