notify: switch to file-based templating system

notify: switch to file-based templating system

Instead of passing the template strings for subject and body when
constructing a notification, we pass only the name of a template.
When rendering the template, the name of the template is used to find
corresponding template files. For PVE, they are located at
/usr/share/proxmox-ve/templates/default. The `default` part is
the 'template namespace', which is a preparation for user-customizable
and/or translatable notifications.

Previously, the same template string was used to render HTML and
plaintext notifications. This was achieved by providing some template
helpers that 'abstract away' HTML/plaintext formatting. However,
in hindsight this turned out to be pretty finicky. Since the
current changes lay the foundations for user-customizable notification
templates, I ripped these abstractions out. Now there are simply two
templates, one for plaintext, one for HTML.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>
Tested-by: Folke Gleumes <f.gleumes@proxmox.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>