appointment model: clean() and save() improvements

[deronnax]

• because of the >= in if self.start_time >= self.end_time:, the if self.start_time == self.end_time: can never be reached
• ValueError is not the right error to raise, It's ValidationError. Raising ValueError throws a 500
• The date field already checks for date validity

Nit: all the checks in the save() should be removed, they are already done in clean() (except one but I can move it into clean). In Django, all checks are done clean, and not in save. The way it's done is contrary to the django philosophy (yes theoretically a silly dev could call save with invalid data because he did not call clean()` but django is clear on it: that's his fault).
If you are OK with this proposal, I will do this.

[adamspd]

@deronnax, you're right about everything said. I merged the pull request and the change wil be in the next release. Thanks a lot for that.

[deronnax]

Cool, thank you. Sorry, I did not notice I broke tests because they did not run on the MR. But I will know for next time.