[Feature] Allow creating PRs from formatted patches

[Seirdy]

Requested feature

git format-patch turns a commit into a patch ready to email. These patches can be applied by the recipient with git am. It would be nice if Gitea could allow uploading a formatted patch to create a PR

Rationale

1. Convenience

Creating a PR with formatted patches simplifies the contribution process.

The existing flow:

1. Fork the repo
2. Clone the fork
3. (optional, recommended) check out a new branch
4. Make commits
5. Push commits
6. Click a link to open an online form for a new PR
7. Fill out the PR form
8. Submit the PR

The flow that this change will allow:

1. Clone the upstream repo (often already done)
2. (optional, recommended) check out a new branch
3. Make commits
4. git format-patch
5. Drag and drop those patch files onto the repo
6. Confirm and submit.

2. Allows contribution without a Gitea account

The best part is that a Gitea admin and repo owner could allow patch files to be submitted without an account, significantly lowering the barrier to entry.

This can also be a big step towards the overall goal of federated PRs by taking the email approach. See #13442 (comment)

[zeripath]

See #10007

Just need some help getting the UI sorted

[SamWhited]

Now that #17902 is merged this is done (you can create a branch from a patch). However, it would be nice to have the ability to do this directly from the new pull request view (where it will be much more discoverable). Maybe this issue can also be used to track the UI from both directions (ie. right now we have the ability to create a pull request from the apply patch view, we should also have the ability to use a patch instead of a branch from the new pull request view)

EDIT: also the feature as merged is slightly different from this request. Only people with commit privileges can apply patches right now; it would be nice if people without could still create PRs or submit the patch some other way for consideration.