Fix series preview table height on the course page

[jorg-vr]

This pull request is a third attempt to fix #3962. See #4167 and #4169 for previous attempts.

This pull request contains two parts:

• A replacement of the old table preview. The new table preview has the correct height in all browser and also contains the activity name as requested by @niknetniko .
• A speed up of the progress bar load, to offset reduced speed by the new table.

Analysis

To analyze the impact of these changes, I looked at two large courses. For fair comparison I disabled memcached and the sql query cache.

http://dodona.ugent.be/nl/courses/800/

102 series | 640 exercises | 488 users | 491.449 submissions

Pr	Total request time	Time in views	Time in Activerecord	Filesize
Old	755ms	620.0ms	125.4ms	833KB
#4167	2049ms	1780.3ms	228.3ms
#4169	6947ms	2827.0ms	4101.8ms
New	1049ms	838.6ms	200.3ms	990KB

http://dodona.ugent.be/nl/courses/359/

22 series | 90 exercises | 623 users | 355.789 submissions

Pr	Total request time	Time in views	Time in Activerecord	Filesize
Old	2006ms	394.3ms	1603.1ms	296 KB
#4167	1931ms	463.9ms	1456.2ms
#4169	1879ms	684.6ms	1182.8ms
New	970ms	352.4ms	607.9ms	326.3KB

Notes

The speed up is much less noticeable for the first course. This is due to the first two series of that course being exams in which few students participated.
In the second course the speedup is much greater then the slowdown, caused by loading the activity names.

In order to limit the increase in file size, I tried to keep the amount of html for each row to a minimum.

Visual

Old

New

Closes #3962 .

PS: this pr also containes some changes to the generated database file.
This is probably caused by me having a backup of production on my machine.
I would keep these changes in as they reflect the state of the production database.

[bmesuere]

Nice work with the index!

• When I deploy this on mestra, the series don't seem to load when they scroll into view (no errors in console, no requests are made).
• I'm not super fond of the difference in height for the skeleton bars. Is there a reason for that? The page grid looks nicer if they are all the same height heights (something in between the 2 heights that are used now)

[jorg-vr]

@bmesuere

1. Should be fixed now. Forgot to correctly set the SKELETON_TABLE_SELECTOR after testing
2. I switched back to the old height. The height difference was to match the height of the elements in the actual table

[pdawyndt]

Feels great to have this issue resolved. Thanks @jorg-vr!