Change the data type for priority in tasks

[AYM1607]

The task model uses ints to encode the priority, this could lead to confusion and bugs.
Change it to use an enum. Add the parsing capabilities needed to interact with firestore.

the enum should look something like:

enum TaskPriority {
    low,
    medium,
    high,
}

[AYM1607]

f381465 resolves this issue.