Close #7570
1. Clearly define the wiki path behaviors, see
`services/wiki/wiki_path.go` and tests
2. Keep compatibility with old contents
3. Allow to use dashes in titles, eg: "2000-01-02 Meeting record"
4. Add a "Pages" link in the dropdown, otherwise users can't go to the
Pages page easily.
5. Add a "View original git file" link in the Pages list, even if some
file names are broken, users still have a chance to edit or remove it,
without cloning the wiki repo to local.
6. Fix 500 error when the name contains prefix spaces.
This PR also introduces the ability to support sub-directories, but it
can't be done at the moment due to there are a lot of legacy wiki data,
which use "%2F" in file names.
Co-authored-by: Giteabot <teabot@gitea.io>
