Use configurable remote name in snippets

[ilya-nurullin]

Closes #19403, and makes it possible to use any remote name in code snippets for an empty repository and pull request.
This change is very helpful to me, because I always use different name for my gitea remote.

Uses setting config module to store the value. Default is origin for backward compatibility.

Screenshots

Empty repo

Pull Request

Settings page

[lunny]

Maybe it should be a per repository setting but not a global one.

[ilya-nurullin]

Maybe it should be a per repository setting but not a global one.

I think we can get started with a global one, and then if needed, make it more precise.
At least, the global one works well for me, and I don't need more granular settings here.

[lunny]

Even if you intend to add a global configuration, please store it in the database rather than in app.ini.

[ilya-nurullin]

Even if you intend to add a global configuration, please store it in the database rather than in app.ini.

Done!

[lunny]

Does the title “Repository Snippets” in the configuration accurately reflect its purpose? It might be more appropriate under the Pull Request Settings section.

[ilya-nurullin]

Does the title “Repository Snippets” in the configuration accurately reflect its purpose? It might be more appropriate under the Pull Request Settings section.

This setting affects two pages at once:

1. Empty repo page - I believe this is the most frequent use case for the setting
2. Pull Request page - you can do a merge using the button on the page, so snippets are hidden by default

Therefore, I would choose a more general name. I am open to any suggestions.