Fix URLJoin, markup render link reoslving, sign-in/up/linkaccount page common data (#36861)

The logic of "URLJoin" is unclear and it is often abused.

Also:
* Correct the `resolveLinkRelative` behavior
* Fix missing "PathEscape" in `ToTag`
* Fix more FIXMEs, and add new FIXMEs for newly found problems
* Refactor "auth page common template data"

modules/markup/render_link_test.go

@@ -18,8 +18,16 @@ func TestResolveLinkRelative(t *testing.T) {
 	assert.Equal(t, "/a/b", resolveLinkRelative(ctx, "/a", "b", "", false))
 	assert.Equal(t, "/a/b/c", resolveLinkRelative(ctx, "/a", "b", "c", false))
 	assert.Equal(t, "/a/c", resolveLinkRelative(ctx, "/a", "b", "/c", false))
+	assert.Equal(t, "/a/c#id", resolveLinkRelative(ctx, "/a", "b", "/c#id", false))
+	assert.Equal(t, "/a/%2f?k=/", resolveLinkRelative(ctx, "/a", "b", "/%2f/?k=/", false))
+	assert.Equal(t, "/a/b/c?k=v#id", resolveLinkRelative(ctx, "/a", "b", "c/?k=v#id", false))
+	assert.Equal(t, "%invalid", resolveLinkRelative(ctx, "/a", "b", "%invalid", false))
 	assert.Equal(t, "http://localhost:3000/a", resolveLinkRelative(ctx, "/a", "", "", true))
 
+	// absolute link is returned as is
+	assert.Equal(t, "mailto:user@domain.com", resolveLinkRelative(ctx, "/a", "", "mailto:user@domain.com", false))
+	assert.Equal(t, "http://other/path/", resolveLinkRelative(ctx, "/a", "", "http://other/path/", false))
+
 	// some users might have used absolute paths a lot, so if the prefix overlaps and has enough slashes, we should tolerate it
 	assert.Equal(t, "/owner/repo/foo/owner/repo/foo/bar/xxx", resolveLinkRelative(ctx, "/owner/repo/foo", "", "/owner/repo/foo/bar/xxx", false))
 	assert.Equal(t, "/owner/repo/foo/bar/xxx", resolveLinkRelative(ctx, "/owner/repo/foo/bar", "", "/owner/repo/foo/bar/xxx", false))