Add programmatical support for downloading source code back

[minhducsun2002]

Closes #76.

Note that #86 will be abandoned due to introduced bug.

EDIT : this PR merely adds the required implementations. It does not propose an UI design.

[dungwinux]

Should return the url directly instead of download it everytime

[minhducsun2002]

should? You shouldn't bridge an API endpoint directly to user.

[dungwinux]

Then, shouldn't it show right in details?

[minhducsun2002]

The details panel is under reconstruction. Including it in is useless nevertheless.

This PR is mainly for adding a feature - not polishing it.

[dungwinux]

Then the feature is not completed, thus, is not approved.

[minhducsun2002]

Then the feature is not completed, thus, is not approved.

Okay, back to the definition of completed - you want me to implement a feature, including debugging it, that I'll definitely throw the implementation away after 48-to-72h in place for a new one? No, thank you. I don't want my effort to be wasted.

And even though we have deadline ourselves, our software isn't as important as the Linux kernel to require a thing called backports.

Something is better than nothing, and the details panel can now be considered a wasted effort. I'll not throw another thing into the void, because even though git reset exists for code, there is nothing like that for the time I've put into it .

Then, shouldn't it show right in details?

What's the connection between linking a blob directly and the place to put the link on the interface? You probably misunderstood somewhere - an endpoint shouldn't be directly linked to the user, and that's it. It does not matter where I have to put it - in a <div> with onClick, a <a>, a <Button>, or whatever.

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And let me state this again : this PR is one in two prerequisites for work on #70 to be initiated. The reason is not that I'm afraid of merge conflicts, but the consequences of it - merge conflicts mean that you have to resolve them by hand, which means, you have to re-write some stuff. And bugs come bundled with code - I will not attempt to debug a snippet that will replace another already tested snippet.

It is a matter of sensible time and energy usage, not that I am lazy to do git commit after git merge.

[dungwinux]

The problem is not that you'll eventually throw code away, but you've assumed lots of things before deciding to code. When implementing, you didn't even ask for the design and just write your thought out without thinking. At least, you can google the design first. I know the UI/UX design can be hard, but it's not okay to keep secret yourself and work on it alone.
Plus, the deadline can be extended, as long as you're able to finish the work in the right way. Don't argue me that you have to git reset because you did it without thinking.

[minhducsun2002]

Fine. This feature will be hidden instead - since you decided not to have it in the table and I don't want to throw my effort into the void, for now, consider the feature a hidden easter egg or something like that - where do I put them anyway? It exists, but not enabled - and that's it.

[minhducsun2002]

Consider this PR to add skeletal things instead of implementing a fully working solution.