[GITEA] Optionally allow anyone to edit Wikis

This is largely based on gitea#6312 by @ashimokawa, with updates and
fixes by myself, and incorporates the review feedback given in that pull
request, and more.

What this patch does is add a new "default_permissions" column to the
`repo_units` table (defaulting to read permission), adjusts the
permission checking code to take this into consideration, and then
exposes a setting that lets a repo administrator enable any user on a
Forgejo instance to edit the repo's wiki (effectively giving the wiki
unit of the repo "write" permissions by default).

By default, wikis will remain restricted to collaborators, but with the
new setting exposed, they can be turned into globally editable wikis.

Fixes Codeberg/Community#28.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
(cherry picked from commit 4b74439922)

models/perm/access/repo_permission.go

@@ -33,6 +33,16 @@ func (p *Permission) IsAdmin() bool {
 	return p.AccessMode >= perm_model.AccessModeAdmin
 }
 
+// IsGloballyWriteable returns true if the unit is writeable by all users of the instance.
+func (p *Permission) IsGloballyWriteable(unitType unit.Type) bool {
+	for _, u := range p.Units {
+		if u.Type == unitType {
+			return u.DefaultPermissions == repo_model.UnitAccessModeWrite
+		}
+	}
+	return false
+}
+
 // HasAccess returns true if the current user has at least read access to any unit of this repository
 func (p *Permission) HasAccess() bool {
 	if p.UnitsMode == nil {
@@ -198,7 +208,19 @@ func GetUserRepoPermission(ctx context.Context, repo *repo_model.Repository, use
 	if err := repo.LoadOwner(ctx); err != nil {
 		return perm, err
 	}
+
 	if !repo.Owner.IsOrganization() {
+		// for a public repo, different repo units may have different default
+		// permissions for non-restricted users.
+		if !repo.IsPrivate && !user.IsRestricted && len(repo.Units) > 0 {
+			perm.UnitsMode = make(map[unit.Type]perm_model.AccessMode)
+			for _, u := range repo.Units {
+				if _, ok := perm.UnitsMode[u.Type]; !ok {
+					perm.UnitsMode[u.Type] = u.DefaultPermissions.ToAccessMode(perm.AccessMode)
+				}
+			}
+		}
+
 		return perm, nil
 	}
 
@@ -239,10 +261,12 @@ func GetUserRepoPermission(ctx context.Context, repo *repo_model.Repository, use
 			}
 		}
 
-		// for a public repo on an organization, a non-restricted user has read permission on non-team defined units.
+		// for a public repo on an organization, a non-restricted user should
+		// have the same permission on non-team defined units as the default
+		// permissions for the repo unit.
 		if !found && !repo.IsPrivate && !user.IsRestricted {
 			if _, ok := perm.UnitsMode[u.Type]; !ok {
-				perm.UnitsMode[u.Type] = perm_model.AccessModeRead
+				perm.UnitsMode[u.Type] = u.DefaultPermissions.ToAccessMode(perm_model.AccessModeRead)
 			}
 		}
 	}