[Data rearchitecture] Handle changes in timeslice durations

[gabina]

What this PR does

This PR adds functionality to change the timeslice duration for a given course. The timeslice duration works at course wiki level as we may want to set different timeslice durations for different wikis within the same course.

There is a new TIMESLICE_DURATION env variable to set the default timeslice duration. In addition, a course may have a timeslice_duration flag to specify timeslice durations for wikis. The flag looks as follows:

:timeslice_duration:
    :default: 86400
    :en.wikipedia.org: 43200
    :es.wikipedia.org: 864000

Lengths are measured in seconds (e.g., 86400 seconds = 1 day, 43200 seconds = 12 hours, 864000 seconds = 10 days).

• If a specific value is defined for a wiki domain (wiki.domain), that value is used for the corresponding wiki.
• Otherwise, the default value is applied.
• If the timeslice_duration flag is not provided for the course, the TIMESLICE_DURATION environment variable is used.

As part of the changes, this PR also stops creating the complete universe of timeslices for course user wiki. That means that we only create a course user wiki timeslice when we found some revisions for that course user wiki in the given period of time. Note that the timeslice could still be empty (revision_count set to 0) because we only consider live revisions (revisions for tracked article courses for which already exists an article record). See CourseUserWikiTimeslice.update_cache_from_revisions. This refactor was addressed in this PR for simplicity reasons.

Open questions and concerns

An admin view to easily get and update the timeslice duration for a course will be created in the future. For now, only manual updates are supported (in the flag field).