feat: subsections

Added subsections as a feature.

Supporting information

• Jira ticket: BB-6964
• GitLab issue: #48 (closed)

Visual changes

(not sure why preview doesn't work, downloaded video works for me, did it via Kazam, PLMK if it won't work)

2022-12-16_opencraft_BB-6964

Merge deadline

None I'm aware of

Testing instructions

1. Create a checklist with subsections via admin UI per screencast attached
2. Either create a checklist from "Templates" page or by setting up a recurrence for a team the current user belongs to
3. Play around with subsections & subtasks in the frontend
4. With a recurrence it's possible to play with tags-based filtering of subsections & subtasks
5. Subsections are excluded from "Trends" & "Compare" reports

Author notes and concerns

Conventional commit message for the squashed commit:

feat: added subsections as a feature

Subsection is a new task type which enables logical/semantic 
separation within checklists. To support this there's a new 
interface type `subsection` and tasks now support parent/child 
relationships. There's intentional limit on just one level of 
nesting for simplicity.

Now if subsection is required it's considered complete if all 
REQUIRED items under it are complete. If subsection isn't 
required then it's considered complete when ALL items under it 
are complete. For now it's expected that authors won't mark 
a subsection required if they're not putting required items 
underneath but overall there's a future UX decision to be made.

Fixes: https://gitlab.com/opencraft/dev/listaflow/-/issues/48