Add support for dotfiles of domains and groups

[yongkyunlee]

• Add dotfiles column for tables domains and groups.
• Add manager API endpoints for listing/getting, creating, updating, and deleting dotfiles.

Solving issue lablup/backend.ai#210.

[codecov]

Codecov Report

Merging #329 into master will decrease coverage by 0.12%.
The diff coverage is 36.36%.

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
- Coverage   48.97%   48.85%   -0.13%     
==========================================
  Files          47       47              
  Lines        6995     7058      +63     
==========================================
+ Hits         3426     3448      +22     
- Misses       3569     3610      +41     

Impacted Files	Coverage Δ
src/ai/backend/gateway/server.py	57.69% <ø> (ø)
src/ai/backend/manager/registry.py	20.02% <0.00%> (-0.60%)	⬇️
src/ai/backend/manager/models/group.py	45.86% <45.45%> (-0.04%)	⬇️
src/ai/backend/manager/models/domain.py	58.57% <52.63%> (-1.03%)	⬇️
src/ai/backend/gateway/exceptions.py	86.05% <100.00%> (+0.20%)	⬆️

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[adrysn]

I left some comments mostly on authorizations.

And, I am worried about what will happen when the name of a dotfile collides with the name of a virtual folder. For example, let's say a superadmin created a dotfile named .test, which will be mounted at /home/work/.test inside a session container. And there is a chance that some users have a virtual folder named .test, which will be auto-mounted. The user tries to launch a session. I'm not sure since I didn't test, but probably the session will not be launched due to naming conflict.

This is not very serious for user's dotfiles since the problem will arise only for that user. But, for domain and group dotfiles, the problem will affect many domains' and/or groups' users, if admins are careless with the dotfile naming.

Currently, I don't have a clear idea to prevent this problem. Let's discuss further with others.

[adrysn]

LGTM. Please merge the master branch into this one, and resolve conflicting file(s). Also, try to fix broken tests if possible.