[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)
(cherry picked from commit 337cf62c10)
(cherry picked from commit b6786fdb32)
(cherry picked from commit a5d2829a10)

[GITEA] Optionally allow anyone to edit Wikis (squash) AddTokenAuth

(cherry picked from commit fed50cf72e)

services/forms/repo_form.go

@@ -132,6 +132,7 @@ type RepoSettingForm struct {
 	// Advanced settings
 	EnableCode                            bool
 	EnableWiki                            bool
+	GloballyWriteableWiki                 bool
 	EnableExternalWiki                    bool
 	ExternalWikiURL                       string
 	EnableIssues                          bool