Refactor upload api call, should be a function in main.BotClient()

DRY

watermark:
https://github.com/ESN-Trondheim/ESNbot/blob/cfc0bd24ae3d89333d7ce434d639fd7794e00cbb/esnbot/commands.py#L178-L180

coverphoto:
https://github.com/ESN-Trondheim/ESNbot/blob/cfc0bd24ae3d89333d7ce434d639fd7794e00cbb/esnbot/commands.py#L253-L255

These should be a function that is a part of `main.BotClient()`.
If we want to do any changes, we will then only have to change the function itself instead of in two places.

Start with esnbot/commands.py at the linked watermark lines 178-180 and coverphoto lines 253-255, then inspect main.BotClient(). Extract the repeated upload API call into a BotClient function and update both locations to use it. Done means upload behavior remains the same while future changes need only one implementation to update.

Written by the indexing model from the issue text.