Sedrila

- ...

- `instructor1` removed.
- `viewer` removed. This functionality is now covered by the 'webapp' function of
`student` and `instructor`.
- `student`: completely rewritten. Except for `--init` and `--import-keys`, the usual form of use is now
menu-driven, with operations 'prepare', 'webapp', 'edit', 'commit', and 'push'.
The webapp also shows a) a list of tasks (based on contents of 'submission.yaml' and commit messages)
and allows selecting/unselecting them for submission, and b) the report on work done so far.
The previous terminal-based task selection and work report were removed.
- `instructor`: rewritten, to make it analogous to `student`.
Here, the webapp allows clicking on usernames near files or tasks in order to accept/reject/keepneutral
the corresponding task. Each such change is reflected in `submission.yaml` immediately.
The work report is available here as well.
- ...

- `viewer`: completely new viewer that can browse several directory trees at once and will
mark most files that pertain to some task given in `submission.yaml`.
- `student`: Five fields are now mandatory in `student.yaml` (in order to support `viewer` and `instructor`):
`course_url`, `student_fullname`, `student_id`, `student_gituser`, `partner_gituser`;
the latter (and only the latter) may be empty.
- `instructor1`: is what `instructor` was formerly, while `instructor` is being re-designed.
- `instructor`: is now a menu-driven interactive command and can also work on several student workdirs at once.
So far, it can only perform pull, viewer, and commit&push, but has no actual task-marking functionality.
INCOMPLETE.
- `student` and `instructor1` are prone to have defects, but `viewer` is useful.
- Documentation "General ideas" now describes the four different directory hierarchies involved.
- Documentation "Architecture" now specifies module layers.
- Ctrl-C now always terminates without a stack trace.

- `author`: Remove the ToC at bottom of homepages of course, chapter, or taskgroup
- `viewer`: Ctrl-C now only prints a nice exit message.
- `viewer`: Show all files named like submitted entries (in any directory) together
- `viewer`: Provide favicon

- `author`: FIX: generate .htaccess again (was missing since 2.0.0)
- `viewer`: make shellprompt recognition more liberal
- `viewer`: allow .prot files to lie in subdirectories rather than at top level

- `instructor`: forgotten FIX: ignore Taskgroups in `require` when checking