Source: Configurable Username

[oxzi]

As proposed in Icinga/icinga-notifications#227, we want to add usernames for sources to enable a more user-friendly authentication for the Icinga Notifications API. At the moment, the username is basically the source ID in the source-${ID} format.

With the proposed change and implementation in Icinga/icinga-notifications#338, each source gets its own unique username.

For the web part, the source UI needs another "listener user" text form, populating the listener_user column of the source table. Details to the schema are available in the diff of Icinga/icinga-notifications#338.