Refactoring: Transform block_opencast from the primary plugin to an optional plugin

[abias]

Recently at the Opencast DACH summit, it was proposed to transform block_opencast from the primary plugin to an optional plugin. This move would allow us to finally fix the most initial but suboptimal design decision (i.e. to make the Moodle-Opencast-interface a block) and to make the Opencast integration into Moodle courses more understandable for end users and admins.

These are the main goals for this issue:

Move settings to tool_opencast

All settings on the pages

/admin/settings.php?section=block_opencast_sharedsettings
/admin/settings.php?section=block_opencast_generalsettings_1
/admin/settings.php?section=block_opencast_appearancesettings_1
/admin/settings.php?section=block_opencast_additionalsettings_1
/admin/settings.php?section=block_opencast_ltimodulesettings_1
/admin/settings.php?section=block_opencast_importvideossettings_1

have to be checked if they control anything about the general Opencast integration behaviour or if they control anything about the presentation in the / of the block.

All settings which control the general Opencast integration behaviour should be migrated to tool_opencast - where they belong to and where all *_opencast plugins in Moodle can use them.

During this migration, extra care has to be spent on (manual and automatic) testing to make sure that existing features do not break.

Likewise, existing settings on existing Moodle installations have to be migrated to tool_opencast with a /db/upgrade.php script.

Move feature pages to tool_opencast

The block's main page /blocks/opencast/index.php as well as all other video management pages below /blocks/opencast should be moved to tool_opencast.

During this migration, the shifted code pieces should remain as untouched as possible to avoid any hickups.
Cleanup candidates, for example settings or features which do not make much sense anymore nowadays, should be documented in follow-up issues for further investigation.

Move the entry point

The entry point to the Opencast management page /blocks/opencast/index.php within a course is currently just provided in the block. This means that the block has to be present in a course and that teachers have to know and understand that Opencast is a block.

The proposed user experience change is to move from this entry process:

to something like this one:

Make the block an optional plugin

Making the block an optional plugin in the end would mean that the block would still exist and could still be added to a course. It would present (for now) the existing videos like it has always done and it could also still allow experienced teachers to enter the Opencast management page if they want to.

However, the block will be an optional plugin in the end and the Opencast integration will also work if only tool_opencast (and mod_opencast for providing videos to viewers) is installed.

Notes

This issue is a comparably large one and the task list might be prolonged as soon as we dive into the specification phase.