edit core & edit event [mutations]

[nagarajpandith]

• write mutations to edit core member and event details on admin panel
• add the mutations editCore and editEvent in /src/server/api/routers/core.ts and src/server/api/routers/event.ts respectively
• note that you also have to handle deleting the old image in cases where admin wants to upload a new image.

  Note while working on this issue, please check if #69 is closed. if it's closed, then you need to handle multiple images for an event here.

[nagarajpandith]

solved by @keerthan-ns in #75