Improve submissions script

[Sv443]

The submissions script was made a lot better in v2.3.2 but it can still be improved:

TODO:

• [x] Enforce strict joke format
• [x] Trim whitespaces at beginning and end
• [x] Prevent duplicates (#345)
• [ ] Add caching (#356)

Bugs:

• [ ] Sometimes when pressing an invalid key the prompt will exit randomly (not a fatal bug but annoying)
• [x] Error when joke file of a submission doesn't exist #350