Refactor repoIndex collection into a DB model

[haishanh]

~still WIP - DO NOT MERGE~

Let me know if you want to collaborate, cause I may squash and force push commits.

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[EDITED]

Ready to merge.

In this PR:

• A new model RepoFileEntry is created to save individual file entry
• Prop tipCommitSha is added to model RepoIndex to track the tip commit of the branch
• A new endpoint /api/repository/updated-collections is added, which will only response modified/removed entries
• Frontend will poll the above API every 8 secs
• Minor code style change(I like semicolon in JS!), I may pretty print all JS files in the future.