Make the social media message translateable by moving the content property to `ExperimentTranslatedContent`

[drikusroor]

See also: https://github.com/Amsterdam-Music-Lab/MUSCLE/issues/1204#issuecomment-2270777135

I have already started this ticket, but I have some things that need to be discussed:

• Should a user be allowed to use points, _experimentname and potential other placeholders in the message?
• Should the social media message always be passed in the get_experiment endpoint and/or should we ideally be able to use it in some get_next_round endpoint with a Final action too?
  • How do we deal with the placeholder values then and their potential non-existence (like no points available)?

Proposed requirements

• Modify individual social media configurations in 4 rules files to get its message from the experiment level
• In the rules base class, get the experiment level's social media message
• Create a simple but elegant way for handling placeholders, including handling singular and plural words like point & points

[BeritJanssen]

I was actually a bit stumped on the issue of placeholders. Django's translation mechanism deals with the placeholders elegantly, and I think it would be quite difficult to achieve this through the database. Possible solutions I can think of now:

• implement the social_media_message & translation in the codebase, perhaps with a couple of options the user can choose from
• let the user upload a .html file which allows placeholders. That would mean we can use the Django translation backend, as the engine also scans the upload folder