Remove PR creation metadata restrictions

[delvh]

• Gitea version (or commit ref): 1.14.0+dev-626-ga7cfb9f2c
• Can you reproduce the bug at https://try.gitea.io:
  • Yes - open any PR.

Description

Currently when opening any PR, the user only has the option to set the milestone,labels and assignees.
He does not have the opportunity to set all other metadata in the creation dialog.
Instead, he has to manually add it after the PR has been created, risking that he simply forgets it (happened to me more than once).
So, what I'm asking for is to give the user also during the creation the ability to add the other metadata
(namely Reviewers, Projects,Due date and Dependencies) - any data not mentioned here
(especially Participants, Notifications and Lock conversation - maybe even Time Tracker) is most likely irrevelant during the creation of a normal PR and can be ignored in the creation dialog.
...

Screenshots

Current sidebar appearance when creating a PR:

Current sidebar appearance after having created the PR:

Expected sidebar appearance during PR creation:

[delvh]

After some digging, I found out that the responsible file for that should be this file.
However, my current knowledge of that file is limited, so for now I'll be unable to fix it.