feat(spx-backend): implement Community API

[aofei]

Fixes #920

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Breaking changes to the API spec

1. Renamed Model.cTime to Model.createdAt.
2. Renamed Model.uTime to Model.updatedAt.
3. Renamed IsPublic to Visibility.
4. Redesigned Visibility as a self-descriptive string enum instead of an integer enum.
5. Renamed Asset.assetType to Asset.type.
6. Redesigned Asset.type as a self-descriptive string enum instead of an integer enum.
7. Changed endpoint /user/following/{username} to /user/{username}/following.
8. Changed endpoint /project/liking/{owner}/{name} to /project/{owner}/{name}/liking.
9. Removed the incorrectly added requestBody.name parameter from PUT /project/{owner}/{name}.
10. Removed the unused preview and clickCount properties from Asset.

[qiniu-ci]

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[aofei]

其他我没问题了

补充了测试 & 同步改动了前端，我本地测试了没啥问题，看看还需不需要再 review 下