replace activities with moments (and add moment cards)

[endoooo]

maybe having one more level of documentation — an activity child — can help in documentation organization.

the use case I'm thinking of is the lesson planning — I think the activity will be used in the weekly planning level, and maybe having a simple children ordered cards can help in the lesson planning.

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thinking about it again, I think I'll propose some naming adjustments:

• activity will be renamed to moment. I think it'll be more on par with the actual use, and it's lack of definition (a moment can be anything, including an activity) is desirable. if we think in the week by week organization, or stages of the project, moment works better than activity. a stage in historical or logical development

• then, for the new feature we can use moment cards. users can (and should!) use the moment description field for instructions, but ordered content blocks can help organize ideas, especially instructions (but I think they can and will be used for other purposes... users have imagination hehe)

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• rename Activity schema to Moment
• create MomentCard schema

[endoooo]

closed by #51