[Offers] Folder Creation

[JamesChandler04]

• When trying to add an offer, if the directory doesn't exist the website crashes.
• Should check if the folders exist and if not, create them.

[maxcutlyp]

os.makedirs(folderpath, exist_ok=True) to avoid having to check first - exist_ok will make it do nothing if the folder already exists. Avoids race conditions where e.g. the folder is created/deleted between checking and creating the folder/saving a file under it (and less lines of code :p)