edit forms, data model & editorial guidelines

a few comments based on the work going on in T7.4 for the editorial guidelines of the Marketplace, that should be added as documentation/static pages for the beta+ version, and also based on my first tests using the edit forms:

1. we are still looking for the best way to structure the guidelines to support contributors and moderators in the creation and review/approbation of the items. One important idea is the need to prioritise attributes/properties depending on the type of items created (i.e. bibliographic metadata are important if you create a publication, maybe less if you create a tool&service), so we came up with this idea of having recommended fields depending on the type of items. This is not part of the data model, but it is probably important to have this reflected in the edit forms, especially for the property section.
2. one other example is the need to adapt the 'step' part of the workflows forms, because even if a step could theoretically supports the same metadata than any other object/entity, only 'label', 'description' and 'related items' are actually displayed on the item view. Based on the discussions we had during T7.4 telcos, we feel a need to also have 'see also' displayed for a step - and maybe even 'media'
3. can we isolate the 'thumbnail' property from the other one in the edit forms? This property doesn't belong to any group ans is displayed in a very different way than the others.
4. we need some clarification regarding the properties recording time (cf. #78)
5. as 'description' allows markdown, we can add links, which is somehow redundant with the purpose of 'see also'. Is it a problem?

notify @edward.gray @nicolas.larrousse @frank.fischer01 @wytrazek @matej.durco @klaus.illmayer @stefan.probst