Move all untis related configuration into the webuntis section.

This is usefull to include events like holidays or semester end in
the formatted output.

I use the following config

```toml
[special_events.Notenschluss]
pattern = "Notenschluss*"
fmt = '''\\rowcolor{{yellow!30}} \\multicolumn{{5}}{{c}}{{{event.start.date().strftime('%d.%m')} {event.summary}}} \\\\'''

[special_events.Ferien]
pattern = "*ferien*"
fmt = '''\\rowcolor{{yellow!30}} \\multicolumn{{5}}{{c}}{{{event.start.date().strftime('%d.%m')}-{event.end.date().strftime('%d.%m')} {event.summary}}} \\\\'''
```

to include any holidays as well as the semester end in my formatted
latex table.

I want to color a table row in a certain color if a early end of class
causes my lesson to be canceled.
To do so I use the new configuration table:

```toml
[event_line_formats]
Unterrichtsschluss = '\\rowcolor{{red!30}} {line}'```