Add API to get/edit wiki (#17278)

Add API to get/edit wiki (#17278)

* Add API to get/edit wiki

* Add swagger docs, various improvements

* fmt

* Fix lint and rm comment

* Add page parameter

* Add pagination to pages

* Add tests

* fmt

* Update func names

* Update error handling

* Update type name

* Fix lint

* Don't delete Home

* Update func name

* Update routers/api/v1/repo/wiki.go

Co-authored-by: delvh <dev.lh@web.de>
* Remove unnecessary check

* Fix lint

* Use English strings

* Update integrations/api_wiki_test.go

Co-authored-by: delvh <dev.lh@web.de>
* Update func and test names

* Remove unsed check and avoid duplicated error reports

* Improve error handling

* Return after error

* Document 404 error

* Update swagger

* Fix lint

* Apply suggestions from code review

Co-authored-by: delvh <dev.lh@web.de>
* Document file encoding

* fmt

* Apply suggestions

* Use convert

* Fix integration test

* simplify permissions

* unify duplicate key Title/Name

* improve types & return UTC timestamps

* improve types pt.2

- add WikiPageMetaData.LastCommit
- add WikiPageMetaData.HTMLURL
- replace WikiPageMetaData.Updated with .LastCommit.Committer.Created

also delete convert.ToWikiPage(), as it received too many arguments and
only had one callsite anyway. sorry for bad advice earlier 🙃

* WikiPage.Content is base64 encoded

* simplify error handling in wikiContentsByName()

* update swagger

* fix & DRY findWikiRepoCommit() error handling

ListWikiPages() previously wrote error twice when repo wiki didn't exist

* rename Content -> ContentBase64

* Fix test

* Fix tests

* Update var name

* suburl -> sub_url

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Norwin <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>