[FEAT]: Allow forking without a repo ID

Forking a repository via the web UI currently requires visiting a
`/repo/fork/{{repoid}}` URL. This makes it cumbersome to create a link
that starts a fork, because the repository ID is only available via the
API. While it *is* possible to create a link, doing so requires extra
steps.

To make it easier to have a "Fork me!"-style links, introduce the
`/{username}/{repo}/fork` route, which will start the forking process
based on the repository in context instead.

The old `/repo/fork/{repoid}` route (with a `GET` request) will remain
there for the sake of backwards compatibility, but will redirect to the
new URL instead. It's `POST` handler is removed.

Tests that used the old route are updated to use the new one, and new
tests are introduced to exercise the redirect.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

templates/repo/header.tmpl

@@ -82,7 +82,7 @@
 									{{/*else is not required here, because the button shouldn't link to any site if you can't create a fork*/}}
 									{{end}}
 								{{else if not $.UserAndOrgForks}}
-									href="{{AppSubUrl}}/repo/fork/{{.ID}}"
+									href="{{$.RepoLink}}/fork"
 								{{else}}
 									data-modal="#fork-repo-modal"
 								{{end}}
@@ -103,7 +103,7 @@
 									</div>
 									{{if $.CanSignedUserFork}}
 									<div class="divider"></div>
-									<a href="{{AppSubUrl}}/repo/fork/{{.ID}}">{{ctx.Locale.Tr "repo.fork_to_different_account"}}</a>
+									<a href="{{$.RepoLink}}/fork">{{ctx.Locale.Tr "repo.fork_to_different_account"}}</a>
 									{{end}}
 								</div>
 							</div>