[Feature/Improvement] Implement skip next/previous

[a-mabe]

Description: As a user, I would like to skip forward to the next interval or skip back to the previous interval.

More details:

With the new IntervalType and ephemeral database holding the list of intervals, skip next/previous will be much easier to implement.

With skip next, the timer should skip to the next interval in the list (increase current interval index by 1). Skip to end of interval if on the last interval.

With skip previous, the timer should skip to the previous interval in the list (decrease current interval index by 1). Restart current interval if on first interval.

Acceptance criteria:

• Skip next and skip previous functions implemented.
• Skip buttons that call the skip functions added to example app.