enhance(ui): use OpenAPI 3.0 spec in API viewer, add spec buttons (#38478)

Follow-ups from https://github.com/go-gitea/gitea/pull/37038:

- Render the OpenAPI 3.0 spec in the API viewer, it is richer than the
Swagger 2.0 rendering
- Replace the back link with gitea-styled buttons to view both specs and
return to Gitea, flowing above swagger-ui on viewports where they would
overlap the title
- Key `VisibilityModes` by the string enum type, now named
`VisibilityString`, removing the `string()` casts at API call sites
- Drop the raw visibility strings from the `Service` settings struct in
favor of the typed mode fields, which also removes the org default
visibility row from the admin config page
- Fix two pre-existing issues surfaced in review: an invalid
`DEFAULT_USER_VISIBILITY` was silently accepted as public, and the org
visibility error message showed the enum zero value instead of the
submitted input

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/admin/config.tmpl

@@ -155,8 +155,6 @@
 					<dt>{{ctx.Locale.Tr "admin.config.default_allow_only_contributors_to_track_time"}}</dt>
 					<dd>{{svg (Iif .Service.DefaultAllowOnlyContributorsToTrackTime "octicon-check" "octicon-x")}}</dd>
 				{{end}}
-				<dt>{{ctx.Locale.Tr "admin.config.default_visibility_organization"}}</dt>
-				<dd>{{.Service.DefaultOrgVisibility}}</dd>
 
 				<dt>{{ctx.Locale.Tr "admin.config.no_reply_address"}}</dt>
 				<dd>{{if .Service.NoReplyAddress}}{{.Service.NoReplyAddress}}{{else}}-{{end}}</dd>