This PR also remove supports for:
1. merge pages when renaming a page to existing page
2. namespaces such as a/b/c
3. nested page such as [[a [[nested page]]]]
Pages merge might be added back depends on RTC, but it should be
decoupled from renaming, otherwise it's too complex.
Namespaces and nested pages have been contributed some critical bugs
that lead data-loss, they're so complex together with page alias,
it's just impossible to have a good test coverage and ensure the app
is stable, especially when page rename and RTC.
