[C2] Restore visual catalogue of courses/sessions in grid mode

[ywarnier]

In the C2 re-implementation of the catalogue, we only have a list of courses (which doesn't handle visibilities correctly yet).

Implement a courses and a sessions catalogues inspired by Chamilo 1.11, with space for:

• title
• image
• duration
• dependencies (sequences)
• price (leave space for that if buycourse is enabled)
• category
• language
• popularity (stars)
• teacher(s)
• subscription button

For sessions, fields are a bit different (duration can be calculated from the sum of courses' duration if any, but if some courses don't have a duration the duration should say the sum with a "+" next to the number). Check 1.11 to complete

[christianbeeznest]

The request is implemented with this PR #6203

Thanks for confirmation.

[ywarnier]

Visual appearance has been restored. Although some of the fields mentioned in the issue description do not appear yet, those remaining elements have been moved to #6235 to avoid cluttering this one.