Optionally number exercises in a series

[bmesuere]

This feature was requested by @IsaacMe in #3411

Change
Add a setting to each series to enable the automatic numbering of exercises.

Result
Where possible, we add automatic sequential numbers to the UI:

• In the series listing on the course page
• In the exercise title when opening an exercise
• In the right hand side quick navigation menu

Limitations

• We don't allow manual numbers, only automatic ones
• This is optional and off by default
• We would number all items in a series. This might cause confusion in series containing both exercises and reading activities. For example, in a series with a reading activity, exercise, reading activity and exercise, "Solve the second exercise" might mean solve the exercise numbered as 2 (i.e. the first exercise) or the actual second exercise being the one numbered as 4.