We need to improve the comments explaining the main ideas / insights of the solution

Regressed in DMOJ/judge-server#673, since now the cache is per-worker-process, and gets destroyed on exit. Will require a rewrite of the caching system.

In #549, we added unittests for dispatch AtCoder and Codeforces.
To increasing test coverage, we should add unittests for other contest (HackerRank, CS Academy, ...).

Not only tests for all other contests, tests for the part of contests are welcome.

A lot of our entities have both a relation (i.e. @ManyToOne, etc.) and a column (i.e. @Column) where we use the latter only because we set a comment on the column.
However, the column comment is almost always 'the x this y belongs to', which is not very useful.
Thus we can remove this duplicate code.

CP Editor supports GUI in multiple languages, including English, Chinese, and Russian. However, the developers don't know every language, and the translators don't always have time, so we need help translating the text.