Drop support for Memos >=0.22 and create a compatibility proxy

[mudkipme]

Due to API instability since the rewrite in Memos 0.22.0, it's difficult for 3rd parties like Moe Memos to catch up with the developments and breaking changes of Memos. Other inactive projects may stuck at the HTTP API in 0.21.0.

Even we catch up the API changes of Memos ASAP, users still have to face a dilemma of having to update both Memos and Moe Memos, and thus may break other 3rd party projects. And some users may use Memos instances of friends, which they are unable to upgrade by themselves, while Moe Memos may got updates automatically from app markets.

Instead of upgrading Moe Memos with breaking changes every time, how about we maintain a compatibility proxy (like a front-end reverse proxy) with necessary 0.21.0 APIs, and versioned it aligned to the Memos release versions. Moe Memos will only talk to this proxy and if a Memos admin decides to upgrade Memos, they can upgrade this proxy simultaneously and existing Moe Memos app (and other 3rd party projects) can just work.

For new features not exists in 0.21.0 the proxy can still provide new APIs but keep its stability.