I made this task into an Epic so we can have subtasks for what we need to do here. Here's how I'm thinking of breaking it up:
1. Frontend for supporting categories as inputs from users. ({T194703})
** Have an accordion for "Categories" under the current "Participants" accordion.
** Tentative mock: {F18229193}
** Notes:
*** We don't need to give them a big textbox here like we did with participants because there will not be as many categories as participants and also we need them to tell us which wiki the category goes with.
*** When there is only one wiki for the event, we can avoid showing them the dropdown for picking wikis entirely. Of course we can keep them if it makes things easier technically.
*** When the user hits ➕, the textbox for category name and the dropdown for picking the wiki appear (similar to how the UI for picking organizers works). The textbox is empty but the wiki dropdown is pre-filled with the one user picked for the last one.
2. Backend for supporting categories as input for users.
** {T194704}
** {T194707}
** {T194709}